The keto/enol tautomerism in acetoacetyl fluoride: properties,

spectroscopy, and gas-phase and crystal structures of the enol formw

Natalya V. Belova,*a Heinz Oberhammer,b Xiaoqing Zeng,c Michael Gerken,zcHelge Willner,

cRaphael J. F. Berger,

dStuart A. Hayes

dand Norbert W. Mitzel

d

Received 9th February 2010, Accepted 28th May 2010

DOI: 10.1039/c002743j

Tautomeric properties of acetoacetyl fluoride, CH3–C(O)–CH2–C(O)–F, were studied by IR

(gas phase), Raman (liquid and solid), and NMR spectroscopy (neat liquid), gas electron

diffraction (GED), X-ray crystallography, and quantum chemical calculations. The keto-enol

tautomer possesses a much higher vapour pressure than the diketo form and therefore the

keto-enol form strongly predominates in the gas phase. In the neat liquid state the thermally

unstable compound tautomerizes at low temperatures slowly to yield the diketo form. Raman and

NMR spectra show equilibrium with strong predominance of the diketo tautomer (>90%) in the

liquid phase. Single crystals, grown at �90 1C from the gas phase, contain exclusively acetoacetyl

fluoride in the keto-enol form and the molecules possess a planar skeleton with overall

CS symmetry and with the O–H bond adjacent to the methyl group.

Introduction

Over the years, b-diketones (b-dicarbonyl compounds of the

type R1C(O)–CH2–C(O)R2) have attracted considerable

interest, because they are important organic reagents and

possess interesting properties.1–3 Keto-enol tautomerization

of b-diketones has been extensively studied both experimentally

and theoretically. It is well known that the equilibrium

between diketo and keto-enol (shortened as enol) is influenced

by the temperature, phase, and solvent.1 Furthermore, the

preference of diketo or enol tautomeric form depends strongly

on the R1 and R2 substituents. In previous investigations4,5 it

was observed that these substituents can be divided into two

groups. Substituents of group I (H, CH3, C(CH3)3, or CF3)

favour the enol tautomer, whereas substituents of group II

(F, Cl, NH2 or OCH3) favour the diketo form. According to

gas-phase structural studies the enol tautomer is strongly

preferred in b-diketones with R1 = R2 = H,6–8 CH3,9–12

C(CH3)3,13 CF3.

14,15 On the other hand, compounds with

R1 = R2 = F,16 Cl,17 OCH3,18 or NH2

5 exist in the gas

phase only in the diketo form.

Of particular interest are the tautomeric properties of

diketones with non-equivalent substituents belonging to

different groups. For such compounds two different enol forms

can occur, with O–H bond close to R1 or R2. Furthermore,

different conformations are feasible for the diketo tautomer,

depending on the relative orientation of the CQO bonds

(see Scheme 1).

In the two dicarbonyl compounds, methyl acetoacetate,

CH3C(O)–CH2–C(O)OCH3, and acetoacetamide, CH3C(O)–

CH2–C(O)NH2, R1 = CH3 belongs to group I and favours

Scheme 1 Enol tautomers (above) and possible conformers of diketo

tautomer of R1C(O)–CH2–C(O)R2 compounds (below). ‘‘s’’ stands

for synperiplanar, sp, (which corresponds to dihedral angles

t(OQC–C–C) of 01 � 301) or synclinal, sc, (t(OQC–C–C) = 601 �301) and ‘‘a’’ for anticlinal, ac, (t(OQC–C–C) = 1201 � 301) or

antiperiplanar, ap, ((t(OQC–C–C) = 1801 � 301).

a Ivanovo State University of Chemistry and Technology, Engelsa av.,7, Ivanovo, 153460, Russia. E-mail: belova@isuct.ru

bUniversity of Tubingen, Institut fur Physikalische und TheoretischeChemie, D-72076 Tubingen, Germany

c Bergische Universitat Wuppertal, FB C – Anorganische Chemie,Gauss Strasse 20, D-42097 Wuppertal, Germany

dUniversitat Bielefeld, Lehrstuhl fur Anorganische Chemie undStrukturchemie, Universitatsstrasse 15, D-33615 Bielefeld, Germany

w Electronic supplementary information (ESI) available: One figureshowing the tautomerization of the diketo into the enol form observedin the gas-phase IR spectra. One figure showing the 13C{1H} NMRspectrum of neat liquid acetoacetyl fluoride. One figure showing theexperimental and calculated modified molecular intensity curves andresiduals at two nozzle-to-plate distances in GED experiment. Onetable showing the GED and calculated interatomic distances,vibrational amplitudes and corrections. One table showing theconditions of GED experiment. CCDC reference number 756446.For ESI and crystallographic data in CIF or other electronic formatsee DOI: 10.1039/c002743jz Permanent Address: Department of Chemistry and Biochemistry,University of Lethbridge, Lethbridge, Alberta, T1K 3M4, Canada.

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the enol form, whereas R2=OCH3 or NH2 favours the diketo

tautomer. The tautomeric properties of these compounds were

studied by different methods. According to 1H and 13C NMR

spectra liquid methyl acetoacetate exists at room temperature

exclusively in the diketo form.19 On the other hand the

vibrational spectra of the liquid were interpreted in terms of

some enol contribution.19 At 423 K, however, a mixture of both

tautomers with the diketo form prevailing was observed in the

liquid (DG0 = G0(keto) � G0(enol) = �1.89(1.61) kcal mol�1).20

Similarly, 1H NMR spectra of gaseous methyl acetoacetate at

temperatures between 377 and 417 K are assigned to a mixture

of both tautomers, resulting in DG0 = G0(keto) � G0(enol) =

0.08 � 1.45 kcal mol�1.20 Unfortunately, the experimental

uncertainty in this NMR study is large, and the result covers a

wide range of tautomeric mixtures of enol/keto in gas phase

from 1 : 13 to 10 : 1. The tautomeric and conformational

properties of gaseous methyl acetoacetate have also been

investigated by gas electron diffraction (GED), which is

supplemented by IR (matrix) spectra and by quantum

chemical calculations.21 The IR (matrix) spectrum confirms

qualitatively the presence of a mixture of enol and diketo

tautomers with the enol form strongly prevailing. GED

resulted in a mixture of 80(7)% enol and 20(7)% diketo forms.

The predictions by B3LYP calculations were in close agreement

with the GED data, whereas the MP2 calculations predicted a

strong preference of the diketo form.

Spectroscopic measurements of an aqueous solution of

acetoacetamide resulted in an equilibrium constant KE =

[enol]/[keto] = 0.11 implying a strong preference of the diketo

form (90%).22 A GED study 4 showed that a mixture of

63(7)% enol and 37(7)% diketo is present in the gas phase.

The tautomeric composition in the gas phase is closely

reproduced by B3LYP/6-31G(d,p) calculations.

In the present study we are interested in the tautomeric and

conformational properties of acetoacetyl fluoride, CH3C(O)–

CH2–C(O)F in the gaseous, liquid, and solid state. In this

b-diketone the substituents R1 and R2 also belong to

different groups. This makes the tautomeric properties of this

compound highly interesting. Because of its low stability at

room temperature, only a single publication reports the

synthesis and properties of acetoacetyl fluoride.23 According

to its IR and 1H NMR spectra, Olah and Kuhn found that

acetoacetyl fluoride exists in the liquid state predominantly in

the diketo form (93% diketo and 7% enol).23

Experimental

Apparatus

Volatile materials were manipulated in a glass vacuum line

equipped with a capacitance pressure gauge (221 AHS-1000,

MKS Baratron, Burlington, MA), three U-traps and valves

with PTFE stems (Young, London, UK). The vacuum line

was connected to an IR gas cell (optical path length 200 mm,

Si windows, 0.6 mm thick) contained in the sample compartment

of the FTIR instrument (Bruker, Vector 22). This arrangement

made it possible to follow the purification process of the diketo

and the enol forms of acetoacetyl fluoride. The final products

were stored in flame-sealed glass ampoules in liquid nitrogen.

By using an ampoule key,24 the ampoules were opened at the

vacuum line, appropriate amounts were taken out and they

were flame sealed again.

Synthesis

The synthesis was carried out based on the reported

procedure23 with minor modifications. As reaction vessel a

PFA container with an internal volume of 15 mL was used (a

PFA needle valve, Galtec, equipped with an 18 cm long PFA

tube (12 mm o.d., 9 mm i.d.), sealed at the bottom). Diketene

(3.0 g, 36 mmol) was transferred in vacuo into the PFA

container. On a stainless steel vacuum line, aHF (1.0 g,

50 mmol) was condensed onto the solid diketene at �78 1C.

The reaction mixture turned slightly red and the container was

agitated at �78 1C until the reaction mixture became a

homogeneous solution. Subsequently the mixture was allowed

to warm to room temperature within two hours. Under

dynamic vacuum the products were separated in three U-traps

held at �30, �60 and �196 1C. The trap held at �60 1C

contained ca. 1.5 g of almost pure acetoacetyl fluoride as

colourless crystals. During repeated purification of the

product, partial loss of product occurred by reaction with

the glass walls forming a yellow product with very low

volatility. The purity of the product was ascertained by

NMR, IR, and Raman spectroscopy to be >98%.

IR and Raman spectroscopy

IR spectra in the range 4000–400 cm�1 of gaseous samples of

the diketo and the enol forms of acetoacetyl fluoride were

recorded on a Bruker Vector 22 spectrometer with an optical

resolution of 2 cm�1 and 32 scans were co-added for each

spectrum. Raman spectra of a liquid sample, flame-sealed in a

glass tube (o.d. 4 mm, i.d. 3 mm), were recorded in the region

4000–50 cm�1 on a Bruker RFS 100/S FT Raman spectro-

meter using the 1064 nm excitation (500 mW) of a Nd:YAG

laser. For each spectrum, 100 scans were co-added with a

resolution of 4 cm�1. For low-temperature Ramanmeasurements,

the most volatile part of the sample was condensed onto a

copper finger at �196 1C in high vacuum.

NMR spectroscopy

The temperature-dependent 1H (400.1 MHz), 19F (376.5 MHz),

and 13C{1H} (100.6 MHz) NMR spectra were recorded on a

Bruker Avance 400 spectrometer. The neat liquid sample was

flame-sealed in a glass tube (o.d. 4.0 mm, i.d. 3.0 mm, length

15 cm) on the vacuum line and placed into a thin-walled 5 mm

NMR tube with some (CD3)2CO as lock. The neat sample of

acetoacetyl fluoride was kept at �40 1C for ca. 1 h prior to the

measurement. The chemical shifts are referenced to external

standards TMS (1H, 13C) and CFCl3 (19F).

X-Ray crystallography

Attempts to obtain crystals from the liquid phase were not

successful. Crystals suitable for single crystal X-ray diffraction

were grown in an L-shaped tube (o.d. 6 mm, length 20 cm).

A small amount (ca. 20 mg) of sample was condensed into one

end at the vacuum line and the other end was flame sealed

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under dynamic vacuum. The end without sample was immersed

into a cold bath at ca. �90 1C, while the whole setup with cold

bath was kept in a refrigerator at �20 1C overnight. The bent

tube was cut in a cold nitrogen stream (ca. �70 1C), and the

colorless crystals were transferred into a trough cooled to

�70 1C by a flow of cold nitrogen. A crystal of acetoacetyl

fluoride having the dimension 0.26 � 0.18 � 0.13 mm3 was

selected under a microscope.

The crystal was centered on an Oxford Diffraction Gemini E

Ultra diffractometer, equipped with a 2 K � 2 K EOS CCD

area detector, a four-circle kappa goniometer, an Oxford

Instruments Cryojet, and sealed-tube Enhanced (Mo) and

the Enhanced Ultra (Cu) sources. For the data collection

the Mo source emitting graphite-monochromated Mo-Karadiation (l = 0.71073 A) was used. The diffractometer was

controlled by the CrysAlisPro Graphical User Interface (GUI)

software.25 Diffraction data collection strategy was optimized

and consisted of 3 o scans with a width of 11. The data

collection was carried out at �123 1C in a 1024 � 1024 pixel

mode using 2 � 2 pixel binning. Processing of the raw data,

scaling of diffraction data and the application of an empirical

absorption correction was completed by using the CrysAlisPro

program.25

The solutions were obtained by direct methods which

located the positions of the non-hydrogen atoms. The final

refinement was obtained by introducing anisotropic thermal

parameters and the recommended weightings for all of the

atoms. The positions of the hydrogen atoms were found in the

difference map and their positions were refined. The maximum

electron densities in the final difference Fourier map

were located near the heavy atoms. All calculations were

performed using the SHELXTL-plus package for the structure

determination and solution refinement and for the molecular

graphics.26

Gas electron diffraction (GED)

The electron diffraction patterns were recorded with the

modified KD-G2 Gasdiffractograph at the University of

Bielefeld 27,28 at two camera (nozzle-to-plate) distances

(25 and 50 cm) with an accelerating voltage about 60 kV.

The sample used for GED study consisted of nearly pure enol

form and was kept at �196 1C. During the GED experiment

the sample was warmed up to �10 1C and the inlet system and

nozzle were at room temperature. The main conditions of the

GED experiment are collected in the Table S2 (ESIw). Thewavelength of electrons was determined from diffraction

patterns of benzene standard. The molecular intensities

sM(s) were obtained in the s-ranges 6.4–31.4 A�1 and

1.6–15.4 A�1 for the short and long nozzle-to-plate distance,

respectively (s = (4p/l)sin y/2, l is electron wavelength and yis scattering angle). The experimental and theoretical intensities

sM(s) are compared in Figure S3 (ESIw).

Results

Quantum chemical calculations

All quantum chemical calculations were performed with the

program set GAUSSIAN 03.29 The structures of the enol and

diketo tautomers of acetoacetyl fluoride were optimized at the

MP2 and DFT (B3LYP) level of theory using either small

(6-31G(d,p)) or large (cc-pVTZ) basis sets. The relative

energies (DE = Eketo � Eenol) and relative free energies

(DG0 = G0keto � G0

enol) obtained with the different computa-

tional methods are summarized in Table 1. According to these

calculations, only the enol form with the O–H bond adjacent

to the methyl group, CH3C(OH)QCH–C(O)F, is a minimum

at the potential energy surface (see Fig. 1). If a starting

geometry with the O–H bond adjacent to the F,

CH3C(O)–CHQC(OH)F, is used, the hydrogen atom flips

over to the acetyl group and the geometry optimization

converges toward the enol form shown in Fig. 1. The same

holds true for methyl acetoacetate and acetoacetamide.4,21

However, for acetoacetyl fluoride a second enol form with

an O–H� � �F instead of the O–H� � �O hydrogen bond is

predicted to be a local minimum on the potential energy

surface and 4.36 kcal mol�1 (B3LYP/cc-pVTZ) higher in

energy and 4.03 kcal mol�1 (B3LYP/cc-pVTZ) higher in Gibbs

free energy. To find all possible diketo conformers the potential

energy surface has been scanned with the B3LYP/6-31G(d,p)

method. The torsional angles t(O1C1C2C3) and t(O2C3C2C1)

were changed in steps of 301 with full optimization of all other

parameters (see Fig. 1 for atom numbering). The existence of

three stable diketo conformers (ac,sp), (ac,ac), (sp,ac) is

predicted (see Scheme 1). For comparison of calculated and

experimental keto-enol equilibrium compositions, Gibbs

free energies were applied, instead of the relative energies.

Differences between DE and DG0 depend primarily on low

frequency vibrations, which differ appreciably for diketo and

enol tautomers. The lowest frequency for the (ac,sp) conformer is

predicted to be 41 cm�1, that of the enol form to be 125 cm�1

(B3LYP/cc-pVTZ). Furthermore, a multiplicity of two has to

be taken into account for the diketo tautomer (both OCCC

dihedral angles may have positive or negative signs). According

to DG0 values (Table 1) both B3LYP and the MP2 calculations

with large basis sets, predict a strong preference of the enol

tautomer. However, at the MP2 level using small basis sets a

preference of the diketo form is predicted.

The structural parameters for the enol tautomer derived

with the B3LYP and MP2 methods and large basis sets

(cc-pVTZ) are listed in Table 2, together with the experimental

results. Vibrational amplitudes and corrections, Dr= rh1 � ra,

were derived (Table S1, ESIw) from calculated force fields

(B3LYP/cc-pVTZ) using Sipachev’s method as implemented

in the program SHRINK.30–32 The vibrational spectra derived

with B3LYP/cc-pVTZ are used for comparison with the

experimental spectra.

Synthetic aspects and properties of acetoacetyl fluoride

Acetoacetyl fluoride was prepared by reacting diketene with

anhydrous HF (aHF) at low temperature as reported by Olah

and Kuhn.23 If the reaction is carried out at room temperature,

only yellow and red decomposition products are formed.

Cleavage of the C–O single bond in diketene by aHF at low

temperatures is a fast reaction and intermediate 1 is formed

(Scheme 2), which in turn isomerizes fast into the diketo 2. The

equilibrium between the two tautomeric forms, the diketo 2

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and the enol 3, is established slowly, within minutes to hours

depending on the temperature, purity, and aggregation state.

At the same time solid, yellow decomposition products are

formed. The decomposition was found to be catalysed by

HF.23 Acetoacetyl fluoride in the enol form 3 was found to

be much more volatile (ca. 20 mbar at room temperature) than

the diketo form 2 and the decomposition products (together

o1 mbar at room temperature). Therefore, by repeated

trap-to-trap condensation on the vacuum line, it was possible

to isolate pure 3 as colourless liquid.

Spectroscopic properties of the enol and diketo tautomers

The slow establishment of the equilibrium between diketo 2

and enol 3 allowed the full characterization of the two

tautomers by IR, Raman, and NMR measurements.

Gas-phase IR measurements suggested acetoacetyl fluoride

exists mainly in the enol form 3, as evidenced by an intense

CQO stretching vibration at 1749 cm�1 for the hydrogen

bridged –C(O)F moiety and a strong CQC stretching

vibration at 1636 cm�1 (Fig. 2, upper trace). The obtained

IR spectroscopic data are listed in Table 3 and show good

agreement with the calculations (B3LYP/cc-pVTZ), for example,

the CQO and CQC stretches are predicted to be strong

IR bands at 1767 and 1658 cm�1, respectively, both

values are close to the corresponding experimental values.

The strong predominance of the enol form of acetoacetyl

fluoride in the gas phase was also confirmed by a GED study

(see below). A Raman sample of the enol form was obtained

by condensation of gaseous acetoacetyl fluoride onto a

cold copper finger (78 K) in high vacuum. The obtained

Raman spectrum is depicted in Fig. 2 (lower trace), the

CQO stretching vibration of the hydrogen bridged –C(O)F

moiety appeared at 1752 cm�1. The Raman spectrum of the

solid showed the enol form as the only tautomer, which

was corroborated by the X-ray crystallographic results

(see below).

Through fast evaporation of the liquid in the vacuum line,

mixtures of both enol 3 and diketo 2 were observed in the gas

phase, as evidenced by the appearance of two CQO stretching

vibrations at 1863 and 1749 cm�1 for the ‘‘free’’ and hydrogen

bridged –C(O)F moieties, respectively (Fig. 3, upper trace),

and also the intense band at 1636 cm�1 for the CQC stretch in

the enol form. The assignment of the first band to the diketo

form was supported by a calculated frequency of 1899 cm�1

for the ‘‘free’’ –C(O)F moiety in the diketo form. By subtracting

the IR bands belonging to the enol form, one can assign the

Table 1 Ab initio MP2 and DFT (B3LYP) optimized dihedral anglest1 and t2, relative energies, Gibbs free energies and abundances of theenol and diketo tautomers of acetoacetyl fluoride

Enol Diketo (ac,sp) Diketo (ac,ac) Diketo (sp,ac)

MP2/6-31G(d,p)t1 (C1C2C3O2) 0.0 112.2 137.1 �10.6t2 (C3C2C1O1) 0.0 2.3 123.5 108.8Erel, kcal mol�1 0.0 1.25 1.44 1.17Grel

0, kcal mol�1 0.0 �1.55 �1.15 �1.40Abundance (%) 3 42 22 33

MP2/cc-pVTZt1 (C1C2C3O2) 0.0 108.8 143.9 �7.3t2 (C3C2C1O1) 0.0 3.1 110.8 103.6Erel, kcal mol�1 0.0 4.87 5.19 5.07Grel

0, kcal mol�1 0.0 1.82 2.22 2.09Abundance (%) 91 4 2 3

B3LYP/6-31G(d,p)t1 (C1C2C3O2) 0.0 120.3 141.3 �10.3t2 (C3C2C1O1) 0.0 6.6 123.2 111.8Erel, kcal mol�1 0.0 5.98 6.36 6.42Grel

0, kcal mol�1 0.0 3.01 3.51 3.48Abundance (%) 98.8 0.6 0.3 0.3

B3LYP/cc-pVTZt1 (C1C2C3O2) 0.0 117.3 147.2 �6.3t2 (C3C2C1O1) 0.0 6.1 109.1 105.2Erel, kcal mol�1 0.0 6.71 7.16 7.06Grel

0, kcal mol�1 0.0 3.62 3.98 3.98Abundance (%) 99.5 0.2 0.15 0.15

Fig. 1 Calculated structure of the enol form of acetoacetyl fluoride.

Fig. 2 IR (upper trace, gas phase, 300 K) and Raman (lower trace,

solid, 78 K) spectra of acetoacetyl fluoride in the enol form.

Scheme 2 Synthetic route to acetoacetyl fluoride.

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bands to the diketo form in the gas phase (Table 4). The IR

spectrum for the diketo form in the gas phase is comparable to

the previous results of the liquid by Olah and Kuhn,23 where

only the diketo tautomer was observed, since they obtained the

liquid product by distillation above room temperature, rather

than by the trap-to-trap condensation on the vacuum line. The

Raman spectrum of the liquid sample, which according to the

NMR measurements (see below) contained more than 90% of

the diketo form, was recorded at room temperature (Fig. 3,

lower trace). This Raman spectrum can be attributed mainly

to 2 due to the observation of a strong CQO stretching

vibration band at 1850 cm�1.

Tautomerization of the diketo into the enol form in the gas

phase was also observed (Fig. S1, ESIw), attempts to follow

this process quantitatively were made but failed due to

decomposition or polymerization with time, which forms

solids on the surface of glass vacuum line. Surprisingly, in

the liquid state the enol form tautomerizes into the diketo

form and the rate of decomposition was sufficiently slow to

allow recording the Raman and NMR spectra.

The 1H, 19F, and 13C{H} NMR spectra of a neat liquid

sample were recorded at �20 1C (no decomposition was

observed at this temperature), which suggested the presence

of both enol and diketo tautomers. According to the integration

of the 19F NMR signals for the two tautomers, a composition of

91.0% diketo 2 and 9.0% enol 3 is obtained. The content of

the enol tautomer decreases to 8.3% and 6.8% at 0 and 20 1C,

respectively. It is assumed that equilibrium has been reached

prior to recording both NMR spectra. The composition

determined at 20 1C is in agreement with the observed

equilibrium reported by Olah and Kuhn.23 As is shown in

the 13C{H} NMR spectrum (Figure S2, ESIw), couplings

between carbon and fluorine were observed for all carbon

atoms in both tautomers, except for the CH3C(O) moiety in

the diketo form (Table 5).

X-Ray crystal structure

Details of the data collection parameters and other crystallo-

graphic information for acetoacetyl fluoride are given in

Table 6, while important bond lengths and bond angles are

listed in Table 2. Acetoacetyl fluoride crystallizes in the

monoclinic space group P21/n. The quality of the low-

temperature X-ray diffraction data was sufficient to refine

the location of all hydrogen atoms. In the crystal structure

acetoacetyl fluoride adopts the enol form 3 (Fig. 4) with a

strong intramolecular hydrogen bond, with O(1)� � �O(2) and

O(2)H(1)� � �O(1) distances of 2.7023(13) and 1.980(19) A,

respectively. The hydroxyl group is bonded to C(3), adjacent

to the methyl substituent. This is paralleled by the difference in

C(1)–C(2) and C(2)–C(3) bond lengths of 1.4180(17) and

1.3509(16) A, respectively. Weaker intermolecular hydrogen

bonds between two adjacent molecules are observed with

O(1A)� � �O(2) and O(2)H(1)� � �O(1A) distances of 3.0041(13)

Table 2 Experimental and calculated structural parameters of acetoacetyl fluoride in the enol forma

Parameters GED (rh1, ah1)b X-Ray crystallography B3LYP/cc-pVTZ MP2/cc-pVTZ

r(C1–C2) 1.436(4) 1.4180(17) 1.430 1.434r(C2–C3) 1.369(4)c 1.3509(16) 1.364 1.362r(C3–C4) 1.495(4)c 1.4795(18) 1.490 1.487r(C3–O2) 1.336(8) 1.3333(15) 1.327 1.329r(C1–O1) 1.208(5) 1.1964(13) 1.209 1.212r(C1–F) 1.347(5) 1.3628(13) 1.352 1.345r(C4–H3) 1.081(6) 0.949(16) 1.086 1.085r(C4–H4) 1.086(6)c 0.948(14) 1.091 1.089r(C4–H5) 1.086(6)c 0.945(15) 1.091 1.089r(O2–H1) 0.982(6)c 0.844(19) 0.988 0.988r(C2–H2) 1.071(6)c 0.914(14) 1.076 1.075r(O1� � �O2) 2.616(10)e 2.7023(13) 2.628 2.614r(O1� � �H1) 1.773(12)e 1.980(19) 1.754 1.727r(O2� � �O1f) 3.0041(13)r(H1� � �O1f) 2.353(18)+C3C2C1 120.5(1.3) 120.77(11) 119.5 119.0+C2C1O1 126.2(1.5) 129.40(12) 127.6 127.3+C2C3O2 121.6(1.0) 123.46(11) 122.8 123.0+O1C1F 119.4(1.0) 117.45(11) 118.9 119.3+O2C3C4 115.1(1.7) 112.98(11) 113.4 113.4+C2C3C4 123.3(1.6)e 123.57(11) 123.8 123.6+C2C1F 114.3(1.5)e 113.15(10) 113.5 113.4+C3O2H1 107.7d 109.7(12) 107.7 106.4+O2H1� � �O1 146.1(2.0)e 143.1(16) 145.4 147.5+H3C4C3 112.8(1.3) 110.6(9) 111.6 111.2+H4C4C3 110.9(1.3) 109.2(9) 109.7 109.5+H5C4C3 110.9(1.3)c 110.5(9) 109.7 109.5+H4C4H3 109.4d 109.2(9) 109.4 109.6+H5C4H3 109.4d 110.5(9) 109.4 109.6

a Distances in A and angles in degrees. For atom numbering see Fig. 1. b rh1—geometrically consistent structure. Uncertainties in

rh1 s = (ssc2 + (2.5sLS)

2)1/2 (ssc = 0,002r, sLS—standard deviation in least-squares refinement), for angles s = 3sLS.c Difference to previous

parameter fixed to calculated (B3LYP/cc-pVTZ) value. d Not refined. e Dependent parameter. f Atom generated by symmetry operation �x + 1,

�y + 1, �z + 2 (in dimer X-ray structure).

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and 2.353(18) A, respectively, resulting in dimer formation in

the solid state (Fig. 5).

Gas electron diffraction (GED) refinement

The experimental radial distribution function (Fig. 6) was

derived by Fourier transformation of the experimental inten-

sities. Model calculations for acetoacetyl fluoride,

CH3C(O)–CH–C(O)F, demonstrate that radial distribution

functions for the enol and diketo tautomers differ very

strongly (see Fig. 6). Differences occur in the region of bonded

distances as well as for non-bonded distances. The experi-

mental curve can be reproduced reasonably well only with the

enol tautomer.

Therefore, in the least squares analysis only the enol form

was considered. The differences between all C–H and O–H

bond lengths and between all C–C bond lengths were

constrained to calculated values (B3LYP/cc-pVTZ) as well

as the orientation of CH3 group (one C–H bond eclipsing the

Table 3 Experimental and calculated vibrational frequencies ofacetoacetyl fluoride in the enol form

Calculateda Experimentalc

Enol IR (gas, 300 K) Raman (solid, 78 K)

n, cm�1 Int.b n, cm�1 Int. n, cm�1 Int.

125 a0 0 o1147 a0 0 o1 142 w184 a0 0 o1 201 w228 a0 4 215 w374 a0 8 377 m422 a0 2 414 m540 a0 12 535 m 537 s588 a0 0 3 575 w 579 m691 a0 3 676 s749 a0 0 9 726 m 770 w808 a0 0 45 791 m 780 w876 a0 0 75 806 m949 a0 49 928 m 929 w,sh973 a0 30 951 m 942 s

1039 a0 8 1029 m1065 a0 0 4.5 1029 w 1040 w1162 a0 128 1139 s 1141 m1238 a0 313 1213 vs 1169 m1401 a0 38 1352 m 1318 m1429 a0 58 1406 m 1371 m1445 a0 105 1417 m 1401 w1472 a0 0 9 1434 m,sh1486 a0 40 1453 m 1452 m1658 a0 429 1636 vs 1599 vs1767 a0 298 1749 vs 1752 vs3041 a0 43092 a0 0 4 2938 vvs3141 a0 9 2982 s3240 a0 o1 3018 m3346 a0 236 3142 m,br 3115 m

a Calculations were performed at the B3LYP/cc-pVTZ level of

theory. b IR intensities in km mol�1. c Relative intensities, abbreviations:

very very strong (vvs), very strong (vs), strong (s), medium strong (m),

weak (w), very weak (vw), shoulder (sh), and broad (br).

Fig. 3 IR (upper trace, gas phase, 300 K) and Raman (lower trace,

liquid, 300 K) spectra of acetoacetyl fluoride as a mixture of diketo

and enol forms. The major bands of the enol form are marked with

asterisks. The strong IR band at 1749 cm�1 (marked with asterisk in

parentheses) is assigned to both enol and diketo forms of acetoacetyl

fluoride.

Table 4 Experimental and calculated vibrational frequencies ofacetoacetyl fluoride in the diketo form

Calculateda Experimentalc

Diketo (ac,sp) IR (gas, 300 K) Raman (liquid, 300 K)

n, cm�1 Int.b n, cm�1 Int.d n, cm�1 Int.

41 550 6123 o1179 6330 2 405 w394 o1 437 w494 3 511 w536 16 595 m578 14 591 w 626 m660 13 699 vs800 1868 14.5 813 m896 18 891 w 869 m1030 41054 39 1031 w1107 251 1107 m 1110 w1186 18 1172 w1252 31 1277 m 1247 m1340 107 1321 m 1314 w1391 43 1374 m1436 22 1432 s1469 171478 161802 176 1749 s 1730 s1899 240 1863 vs 1850 s3033 33040 2 2934 vs3095 3 2927w,sh 3102 1

3015 vw,sh3146

5

a At the B3LYP/cc-pVTZ level of theory. b IR intensities in km mol�1.c No IR or Raman spectrum of acetoacetyl fluoride in the pure diketo

form was obtained, thus the major IR (gas phase) and Raman (liquid)

bands of the diketo form were obtained by subtracting the correspond-

ing spectra of the enol form. d Relative intensities, abbreviations: very

strong (vs), strong (s), medium strong (m), weak (w), very weak (vw),

shoulder (sh), and broad (br).

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CQC double bond). According to quantum chemical calculations

a planar skeleton with overall CS symmetry was assumed.

Starting parameters from B3LYP/cc-pVTZ calculation were

refined by a least-squares procedure of the molecular intensities.

Geometrically consistent rh1 parameters were used to describe the

molecular structure. Vibrational amplitudes were refined in

groups with fixed differences. With the above assumptions five

bond distances and seven bond angles (Table S1, ESIw) were

refined simultaneously together with seven groups of vibrational

amplitudes. Three correlation coefficients had values larger than

|0.7|: (r(C3–O2)/(+C2C3O2) = �0.89, (r(C3–O2)/(r(C1–F)) =

�0.73, (r(C1–O1)/r(C1–F)) = 0.79.

The best agreement factor resulted for 100% enol form with

Rf 3.8%. Several least squares refinements were performed for

mixtures of enol and diketo tautomers. Rf factors of 4.0 and

4.5% were obtained for 2 or 5% contributions of the (ac,sp)

diketo conformer, 4.1 and 4.7% for 2 or 5% contributions of

the (ac,ac) diketo conformer and of 4.0 and 4.5% for 2 and 5%

Table 5 NMR spectroscopic data of neat acetoacetyl fluoride at �20 1Ca

Diketo form (91.0%) Enol form (9.0%)

1H dH(CH3, s) = 1.9 ppm dH(CH3, s) = 1.7 ppmdH(CH2C(O)F, d) = 3.6 ppm, 3J(HF) = 6.6 Hz dH(CHC(O)F, d) = 4.8 ppm, 3J(HF) = 5.8 Hz

dH(CH3C(OH)) = 8.9 ppm19F dF(CH2C(O)F, t) = 48.8 ppm, 3J(FH) = 6.5 Hz dF(CHC(O)F, dd) = 23.2 ppm,

3J(FHC) = 6.1 Hz, 5J(FHO) = 3.5 Hz

13C{1H} dC(CH3, d) = 30.1 ppm, 4J(CF) = 3.7 Hz dC(CH3, d) = 21.6 ppm, 4J(CF) = 5.9 HzdC(CH3C(O), s) = 203.1 ppm dC(CH3C(OH), d) = 184.1 ppm, 3J(CF) = 18.3 HzdC(CH2C(O)F, d) = 48.2 ppm, 2J(CF) = 52.0 Hz dC(CHC(O)F, d) = 85.9 ppm, 2J(CF) = 64.4 HzdC(CH2C(O)F, d) = 159.0 ppm, 1J(CF) = 357.9 Hz dC(CHC(O)F, d) = 164.5 ppm, 1J(CF) = 327.1 Hz

a Abbreviations denote singlet (s), doublet (d), triplet (t), doublet of doublets (dd).

Fig. 4 View of one acetoacetyl fluoride molecule in the enol form

in the crystal structure, thermal ellipsoids are drawn at the 50%

probability level.

Fig. 5 Hydrogen-bonded dimer of acetoacetyl fluoride in the enol

form, thermal ellipsoids are drawn at the 50% probability level.

Symmetry code: (i) �x + 1, �y + 1, �z + 2.

Table 6 Summary of crystal data and refinement results for aceto-acetyl fluoride in the enol form

Chemical formula C4H5FO2

Space group P21/n (No.14)a/A 5.2191(3)b/A 10.1429(5)c/A 9.1807(5)a (1) 90b (1) 98.606(5)g (1) 90V/A3 480.53(7)Z (molecules/unit cell) 4Mol wt 104.08Calculated density/g cm�3 1.439T/1C �123m/mm�1 0.138R1

a 0.0320wR2

b 0.0720

a R1 is defined as S||Fo| � |Fc||/S|Fo| for I > 2s(I). b wR2 is defined as

[S[w(Fo2 � Fc

2)2]/Sw(Fo2)2]

12 for I > 2s(I).

Fig. 6 Experimental (dots) and calculated radial distribution

functions and difference curve for acetoacetyl fluoride in the enol

form. The calculated radial distribution functions for the diketo form

are also shown for comparison.

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contributions of the (sp,ac) diketo conformer. Using Hamilton’s

criteria 33 with 99% significance level we obtain a tautomeric

composition of 100(3)% enol. Final results of the least squares

analysis for the enol tautomer are given in Table 2 (structural

parameters) and Table S1 (vibrational amplitudes). The

refined structural parameters are rather similar to those

predicted by the quantum chemical calculations.

Discussion

As pointed out in the Introduction, acetoacetyl fluoride,

CH3C(O)–CH2–C(O)F is a b-diketone with the substituents

R1 and R2 belonging to different groups analogous to methyl

acetoacetate, CH3C(O)–CH2–C(O)OCH3, and acetoacetamide,

CH3C(O)–CH2–C(O)NH2. For the last two compounds a

mixture of both tautomeric forms with the predominance of

enol was observed in the gas phase. Thus, a tautomeric

mixture with predominance of the enol form is also expected

for acetoacetyl fluoride in the gas phase. However, according

to GED only the enol tautomer is present in the gas phase.

Gas-phase IR spectra also confirm the enol to be the most

stable tautomeric form. Only occasionally, a few percent of the

diketo form were present. In contrast to methyl acetoacetate

and acetoacetamide, no significant amounts of the diketo form

were found for gaseous acetoacetyl fluoride. These results are

in excellent agreement with the predictions by quantum

chemical calculations. In contrast to most calculations, the

MP2/6-31G(d,p) method predicts a preference of the diketo

tautomer. Previous studies of tautomeric properties of

b-diketones 4,18,34 also demonstrate, that the MP2/6-31G(d,p)

approximation strongly underestimates the enol contribution

compared to results derived from GED experiments and higher-

level calculations. Thus, the MP2/6-31G(d,p) approximation

fails to predict the tautomeric properties of b-diketones.Surprisingly acetoacetyl fluoride adopts the diketo form in

the liquid state and the enol form in the gaseous state. The

preference of the diketo form in the liquid state can only be

explained by stronger intermolecular forces in the liquid state

of the diketo tautomer as compared to the enol. The stronger

intermolecular forces of the diketo form are also reflected in its

significantly lower vapour pressure. The equilibrium composition

of the liquid contains a small amount of the enol tautomer.

According to temperature-dependent NMR studies the

concentration of the diketo form in the liquid increases from

91% at �20 1C to 93.2% at 20 1C. This increase of the main

component with increasing temperature can be rationalized by

a large entropy difference DS = Sdiketo � Senol. Indeed,

our quantum chemical calculations demonstrate that the

vibrational contribution to this entropy difference is positive

and about 6 cal/(mol*K) (B3LYP/cc-pVTZ, MP2/cc-pVTZ).

Since the liquid sample used in the GED experiment consisted

predominantly of the diketo tautomer, the observation of

essentially the pure enol form in the gas phase can be

rationalized by a much higher vapour pressure of the enol

tautomer compared to the diketo form at �10 1C. Since slow

establishment of the equilibrium between diketo and enol in

the gas phase was observed in the IR spectra (see above), it is

very unlikely that fast establishment of this equilibrium occurs

in the GED experiment.

The gas-phase structure of acetoacetyl fluoride obtained

from the gas electron diffraction is in excellent agreement with

the crystal structure. The hydroxy group is located on the side

of the methyl group, while the fluorine is linked to the

carbonyl group. This conformer is predicted, since methyl

and fluorine substituents belong to group I and II, respectively.

A Natural Bond Orbital (NBO) analysis for b-diketoneswith different substituents R1 and R2, including acetoacetyl

fluoride,35 demonstrates that a strong orbital interaction

between an electron lone pair of fluorine and the antibonding

p*(CQO) bond stabilizes the FCQO structure. The gas-phase

and solid-state structures are essentially identical, with minor

differences, which can be due to packing effects, the absence of

dimers in the gas phase, or systematic differences between solid

state and the gas phase distances. In the solid state distances

between vibrationally averaged atom positions are determined,

whereas in the gas phase vibrationally averaged distances are

observed. The most prominent difference between the two

structures is the O1� � �O2 distance which is considerably longer

in the crystal (2.7023(13) A) than in the gas phase (2.616(10)

A), indicating weakening of the O–H� � �O intramolecular

hydrogen bond. This weakening is expected due to formation

of dimers in the crystal with intermolecular hydrogen bonds.

Acknowledgements

This work was supported by the Deutsche Forschungsge-

meinschaft, the Fonds der Chemischen Industrie and by the

Russian-German Cooperation Project (Russian Foundation

for Basic Research Grant N 09-03-91341-NNIO_a and DFG

436 RUS 113/69/0-7). N.V.B. is grateful to the Deutsche

Akademische Austauschdienst (DAAD) and Russian Ministry

of Education and Science for a fellowship of her visit to

Germany. X.Z. acknowledges a fellowship from the Alexander

von Humboldt Foundation and M.G. acknowledges the

University of Lethbridge for granting a study leave. We are

indebted to Dr Jurgen Stohrer and Dr Elke Fritz-Langhals

(Wacker Chemie AG) for providing us with a sample of

diketene and also to Dr Johannes Eicher (Solvay Fluor

GmbH) because he donated to us a steel cylinder with aHF.

We also acknowledge Prof. Eujen for NMR measurements

and helpful discussions.

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