International Journal of Mass Spectrometry and Zon Processes, 94 (1989) 221-235 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

221

A STUDY OF THE AMINOMETHYL RADICAL CH,NH, BY VACUUM UV PHOTOELECTRON SPECTROSCOPY AND AB INITIO MOLECULAR ORBITAL CALCULATIONS

J.M. DYKE *, E.P.F. LEE and M.H. ZAMANPOUR NIAVARAN

Department of Chemistry, The University, Southampton SO9 5NH (Gt. Britain)

(Received 30 March 1989; in final form 8 May 1989)

ABSTRACT

Ultraviolet photoelectron spectra have been recorded for the reaction of atomic fluorine with methylamine at different reaction times. A band associated with a short-lived reaction product has been recorded with adiabatic and vertical ionization energies of 6.29kO.03 and 6.97kO.03 eV respectively. Two vibrational series with average separations of 181Ok 50 and 1190 f 50 cm- were observed in this band. Comparison between the experimental ionization energies and ionization energies computed for CH,NH,( X 2,4) and CH,NH( X *A), in addition to inspection of computed vibrational frequencies of CH,NH,+( XA,) and CH,NH+( X3A) led to assignment of the photoelectron band to the ionization process CH2NH2+ (X A,) +- CH,NH,( XA). The larger vibrational series was assigned to excita- tion of the C-N stretching mode in the ion while the smaller series was assigned to excitation of a CH, out-of-plane vibration. The experimental adiabatic ionization energy has been combined with the known heat of formation of CH,NH,(X*A), to yield the heat of formation A Hrgq8) of CH,NH,+(XA,) as 755&11 kJ mol-.

INTRODUCTION

Aminoalkyl radicals play an important role in the free radical chemistry of biological systems and are formed in amine-inhibited free radical reac- tions [1,2]. As part of a programme to study the electronic structure of small free radicals with UV photoelectron spectroscopy, the first photoelectron band of CH,NH,, the simplest a-aminoalkyl radical, has been recorded. Assignment of this band has been assisted by appropriate ab initio molecu- lar orbital calculations.

* Author to whom correspondence should be addressed.

016%1176/89/$03.50 0 1989 Elsevier Science Publishers B.V.

222

The substituted alkyl radical CH,NH, is noteworthy in that it is expected to have a low first adiabatic ionization energy; 6.1 + 0.2 eV has been determined indirectly by electron impact mass spectrometric experiments on a number of ethylenediamines [3] and 5.86 eV has been derived from ab initio ASCF calculations [4]. These values compare with the first adiabatic ionization energies of CH,, CH,F, CH,OH and CH,CH, determined by photoelectron spectroscopy of 9.840 It 0.005 eV [5], 9.04 + 0.01 eV [6], 7.56 f 0.01 eV [7,8] and 8.26 + 0.02 eV [9].

Ab initio molecular orbital calculations have been performed previously on CH,NH, and CH2NHZf at various levels of sophistication [lo-181 and it is clear from these calculations that CH2NH2+ in its ground electronic state (A,) is planar with C,, symmetry [lo-131. In contrast CH,NH, in its ground state is pyramidal at the radical centre, adopting a chair conforma- tion of C, symmetry [14-181. The other main geometry change between the molecule and the ion indicated by these calculations is that the C-N equilibrium bond length decreases on ionization. These changes in equi- librium geometry will obviously affect the shape and structure of the CH,NH, first photoelectron band.

EXPERIMENTAL

The aminomethyl radical was prepared in this work via the rapid gas phase reaction of fluorine atoms with methylamine [19]. Methylamine was obtained in the gas phase by pumping on a solution of methylamine in water (BDH [20], 25% w/v). Contributions of water to the methylamine photoelec- tron spectra were reduced to negligible levels by pumping the vapour above this solution through a trap cooled by an ice/salt mixture placed between the flask containing the methylamine solution and the spectrometer inlet valve. He1 photoelectron spectra obtained in this way for methylamine were found to be identical with that reported in the literature [21]. Fluorine atoms were produced from a microwave discharge (2.45 GHz) of 5% molecular fluorine in flowing helium. The fluorine atom yield, as judged from the spectra, was at least 90% in all cases.

Spectra were recorded using both a single-detector and a multidetector photoelectron spectrometer that had been specifically designed for the study of short-lived molecules in the gas phase [22,23]. All spectra were recorded with He1 radiation (21.22 eV). Spectral calibration was achieved using the HeI, spectra of ethyl iodide, oxygen and hydrogen fluoride, as well as the band associated with the He11 (40.81 eV) ionization of helium. Under the experimental conditions at which the CH,NH, spectra were obtained, the resolution was 25-30 meV as measured (full width at half maximum) for argon ionized with He1 radiation.

223

COMPUTATIONAL DETAILS AND RESULTS

As the CH,NH, and CH,NH radicals are both expected to be produced as primary products of the rapid F + CH,NH, reaction [19], ab initio molecular orbital calculations were performed on both of these radicals to assist the assignment of the experimental photoelectron spectra. The general strategy used in the calculations was first to compute the minimum energy geometries at the SCF level of both of these radicals in their ground electronic states. Once this was achieved vertical ionization energies, which include the effects of electron correlation, could be computed by performing single-point calculations on the neutral and low-lying ionic states obtained by one-electron ionization from the neutral molecule, at the computed neutral molecule geometry. Geometry optimizations were also performed on the low-lying electronic states of CH2NH2+ and CH3NH+. The optimized molecular and ionic geometries were used in analytical second derivative calculations to yield harmonic vibrational frequencies.

Calculations were carried out using the quantum chemistry programs GAMESS [24], CADPAC [25] and ATMOL [26]. The basis set used, which was of triple-zeta plus polarization quality (TZVP), consisted of the con- tracted Gaussian basis set of Dunning [27] ([5s3p] for carbon and nitrogen and [3s] for hydrogen) augmented with polarization functions (these were C d(exp. 0.72); N d(exp. 0.98) and H p(exp. 1.00) [28]). All SCF calculations were of the restricted Hartree-Fock type.

Initially full geometry optimizations were performed for CH,NH and CH,NH, in their ground states using analytical first derivatives of the SCF energy with respect to nuclear displacements, as implemented in the GAMESS program. Initial geometries were taken from those obtained by Lathan et al. [29] computed at the SCF level with an STO-3G basis set. Once an optimized geometry was obtained, further refinement was achieved with the CADPAC code using the Schlegel method [30] with tightened SCF and gradient convergence tolerances. Each optimized geometry was then used in an analytical second derivative calculation [31] using CADPAC. This gave harmonic vibrational frequencies for CH,NH,( X *A) and CH,NH( X *A). The optimized geometries and computed total energies obtained from these calculations are shown in Table 1 and the computed frequencies are pre- sented in Tables 2 and 3.

The electronic configurations of CH,NH,( X *A) and CH,NH( X *A) at their optimized geometries are

- - - - (5&)*(2&)*(6a)*(7a) CH,NH,( X *A)

- - - - (l~)2(6u)2(7a)2(2~)1 CH,NH( X *A)

TA

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Com

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(deg

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(deg

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3980

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2604

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1.08

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C

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1.36

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107.

26

107.

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0205

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6.43

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0.99

52

115.

80

1.00

27

119.

91

114.

04

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40

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95

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5(H

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7.71

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121.

54

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69

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69

59.6

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14

3.68

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225

TABLE 2

Computed harmonic vibrational frequencies (cm-) of CH,NH,( X*A) and CH2NH2+

(XA,) a

CH,NH,( X*A) b CH2NH2+ (XA,) bsc CH2NH2+ (XA,) d-e

a 3775 3268 1810 1599 1247 869 752

,I a 3870 3375 1445 1018 433

al 3708(3315) 3529(3510) 3304(2995) 3224(3240) 1916(1721) 1788(1799) 1726(1548) 1620(1626) 1557(1397) 1492( 1496)

a2

b,

1139(1022) 1104(1107) 1304(1169) 1159(1185) 1013 (909) 920 (975)

b, 3817(3410) 3642(3617) 3424(3104) 3356(3369) 1464(1310) 1368(1376) 1038 (929) 977 (986)

a See text for details of these calculations. b SCF calculated frequencies. Values in parentheses are the HF/6-31G * values of ref. 36 uniformly scaled by 0.89. d SCF/MP2 calculated frequencies. e Values in parentheses are the MP2/6-31G* values of ref. 36.

TABLE 3

Computed harmonic vibrational frequencies (cm-) of CH,NH(X*A) and CH3NH+ (X3A) a

CH,NH( X *A) CH3NH+(X3A)

a 3659 3628 3249 3301 3140 3180 1611 1552 1558 1543 1435 1222 1131 945 1071 762

I! a 3185 3286

1622 1557 1073 1156 296 169

a See text for details of these calculations. The values quoted are SCF calculated frequencies in cm-.

226

TABLE 4

Computed vertical ionization energies (ev) for CH,NH and CH,NH, a

Ionization process

CH2NH2+X?4 +- CH,NH, X2/f CH NH +u3A+- CH,NH, X2,4 CH:NH:+ XA, + CH,NH, X2,4 CHaNH+ X3,4, CH,NH X2/l CH,NH+ a A + CH,NH X2A

ASCF

6.83 9.55 5.55 c 9.23

10.16

ASCF+ Experimental CI+Q b (this work)

6.88 6.97 f 0.03 10.83 _

5.86 6.29 + 0.03 d 10.16 - 10.45 -

a See text for details of these calculations. b ASCF value plus allowance for correlation energy correction by configuration interaction calculations including all single and double excitations in each state with Davidsons correction [33,34] to the total energy of each state. For CH,NH, , + its optimized geometry was used instead of that of CH,NH,(XA). These values are therefore computed adiabatic ionization energies. d Experimental adiabatic ionization energy of CH,NH,.

For CH,NH,( X2,4), ionization from the 7a level will give a A state whereas ionization from the 6a level gives 3A and A states. Similarly, for CH,NH( X*A), the (2a)- ionization gives a A state whereas the (7a ) - ionization gives 3A and A states. For CH2NH2+, as the 3A state arising from the (6a)(7a) configuration is expected to lie lower in energy than the (6a)(7a) A state, SCF calculations were only performed on the (6a)(7a) 3A and the (6a)*(7a) A states. Also, for CH,NH+ only the (7a)(2a) 3A and (7a)*(2a) A states were considered. By carrying out SCF calculations on these states as well as on the neutral ground state at the computed neutral molecule geometries, ASCF vertical ionization en- ergies were obtained (see Table 4). The computed total energies for each state were then corrected for the effects of electron correlation via configura- tion interaction calculations. The SCF calculations were performed using the ATMOL3 suite of programs [26] and the configuration interaction calcula- tions used the ATMOL Direct-C1 method [32]. All configurations generated by single and double excitations with respect to the reference configuration were considered in the configuration interaction procedure with the con- straint that the two lowest lying doubly filled molecular orbitals (essentially the Cls and Nls levels) were held frozen. In each case it was found that the reference configuration was the main contributor to the wavefunction of the state in question, with a coefficient greater than 0.95. Quadruple excitations were allowed for by applying Davidsons correction [33,34]. The results of these calculations are presented in Table 4. For the ionic states considered, optimized geometries and vibrational frequencies were computed using the procedure already described. The computed vibrational frequencies for

227

CH,NH,+( X A,) are presented in Table 2. From the total energies com- puted at the optimized geometries for CH,NH,( X*A) and CH2NH2+- (X Ai), the first adiabatic ionization energy of CH,NH, has also been calculated (see Table 4).

For the CH,N and CH4Nf states considered, the SCF optimized geome- tries and computed total energies are presented in Table 1. At the SCF level CH,NH,( X*A) is computed to lie 3 kJ mol- lower in energy than CH,NH( X A). After allowance for electron correlation via configuration interaction calculations at the SCF optimized geometries, this difference is modified to 20 kJ mol- again with CH,NH,( X *A) being the more stable. These results are consistent with the experimental heats of formation of these radicals which indicate that CH,NH,(X*A) is 28 It 16 kJ mol- more stable than CH,NH( X *A) [35]. For CH,NH,+ the lowest lying state is the closed-shell A, state in C,, symmetry arising from the configuration . . . (5a1)*(2b2)*(lW2 whereas for CH3NH+ the lowest state is the triplet state 3A, arising from the configuration . . . (6a)*(7a)(2a). The CH2NH2+( X Al) state is computed via SCF calculations to lie 286 kJ mol- lower than the CH,NH+( X 3A) state at their respective SCF optimized geometries. This value compares with an energy difference of 249 kJ mol- computed via SCF calculations with a 4-31G basis set using STO-3G optimized geometries [29].

The SCF optimized geometries for CH,NH(X*A) and the CH,NH+ states, X 3A and a Al, are listed in Table 1. All of these states possess a C, staggered equilibrium geometry (see Fig. l(d)) and for the CH,NH+( X 3A) + CH,NH( X *A) ionization, the major equilibrium geometry change is expected to be an increase in the CNH angle at the nitrogen centre. Similarly, CH,NH,( X 2A) and CH2NH2+( a 3A) possess a C, minimum energy geometry (see Figs. l(a) and l(c)). In contrast CH2NH2+( XA,) is computed to have a planar C,, minimum energy geometry (Fig. l(b)). As well as this geometry change, the other major geometry change that occurs for the CH2NH2+( X Ai) +- CH,NH,( X*A) ionization is in the C-N bond length which is computed at 1.398 A in CH,NH,( X *A) and 1.260 A in CH2NH2+( X A,) at the SCF level (see Table 1). Although the computed SCF minimum energy geometries shown in this table are consistent with the results of previous SCF calculations performed for these molecules and ions with similar quality basis sets [lo-181, the results are presented here as they have all been obtained with the same basis set.

As stated earlier, force constant calculations have been performed for the CH,N and CH4N+ species listed in Table 1. However, only the computed harmonic frequencies for the ground electronic states of CH,NH,, CH2NH2+, CH,NH and CH,NH+ are presented in this work (see Tables 2 and 3). In general, it was not possible to compare all the results in these

228

c 2v H1

\ /

H3

/ C-N

\ H2 H4

lb)

cs H24 /

C-N

/ "1

(d)

Fig. 1. Computed minimum energy structures for some states of CH,NH,, CH2NH2+, CH,NH and CH,NH+ (see Table 1).

tables with previous calculations as computed harmonic frequencies for all four states have not previously been published. However, the results ob- tained for CH2NHzf( X rA1) are very similar to those obtained by DeFrees and McLean [36] at the SCF level using a 6-31G * basis set (see Table 2).

In the case of CH,NH,(X A,) the minimum energy geometry and vibrational frequencies were also computed using the perturbation approach for inclusion of electron correlation known as the Moller-Plesset method [37]. This perturbation correction was implemented at second order (MP2) using the CADPAC program [25]. As can be seen from Table 2, the agreement with the results of DeFrees and McLean [36] calculated by the same method but with a 6-31G* basis set is good.

RESULTS AND DISCUSSION

He1 photoelectron spectra were recorded for the F + CH,NH, reaction at mixing distances above the photon beam of O-20 cm, which correspond to approximate reaction times of O-20 ms. Figure 2 shows three spectra in which the partial pressure of the reagents, F2 and MeNH,, have been held constant. Figure 2(B) and 2(C) were recorded at 2 and 4 cm above the photon beam whereas Fig. 2(A) was recorded for undischarged molecular fluorine mixed with methylamine. Figure 2(C) consists of bands associated with HF [38], HCN [39] and CH,NH [40], which are reaction products, whereas Fig. 2(A) consists of bands arising from F2 [41] and CH,NH, [21]. In Fig. 2(B) a reaction product band was observed centred at approximately

229

7.0 eV ionization energy. This band was clearly associated with a short-lived molecule as increasing the mixing distance above the photon beam caused it to disappear whereas bands due to CH,NH and HCN increased in intensity. A mixing distance study performed at constant reagent partial pressure indicated that the first band of CH,NH, decreased with increasing mixing distance, the band at approximately 7.0 eV ionization energy maximized at 0.75 cm, the first band of CH,NH ionization energy maximized at slightly longer mixing distances and HCN increased from zero to reach a constant level at approximately 9.0 cm mixing distance. The band at 7.0 eV was consistently observed to maximize in concentration in the reaction sequence at shorter reaction times than CH,NH and the molecule associated with this feature almost certainly reacts with fluorine atoms to produce CH,NH and subsequently HCN. On the basis of this evidence the band at 7.0 eV is assigned to ionization of CH,N produced as a primary product of the F + CH,NH, reaction. In this reaction two primary hydrogen atom abstrac- tion reactions are possible

F + CH,NH, + CH,NH, + HF AH, = -179 + 11 kJ malli (1)

F + CH,NH, + CH,NH + HF AH, = -151 + 11 kJ mol- (2)

Use of experimental heats of formation of CH,NH, and CH,NH [35,42,43] in addition to those of F, HF and CH,NH, [44,45] shows that reaction 1 is about 28 kJ mol- more exothermic than reaction 2.

An expanded scan of the band centred at approximately 7.0 eV in Fig. 2(b) is shown in Fig. 3. The adiabatic and vertical ionization energies of this band were measured as 6.29 + 0.03 and 6.97 + 0.03 eV respectively. Also, evidence of a regular vibrational series was observed in this band which had an average vibrational separation of 1810 &- 50 cm-.

In most spectra of the type shown in Fig. 3 a second series with lower vibrational frequency was observed and this was measured to give an average separation of 1190 + 50 cm-. As this band has already been attributed to ionization of either CH,NH, or CH,NH, comparison of the observed vertical ionization energy with the values computed in this work for the first vertical ionization energies of CH,NH, and CH,NH (6.88 and 10.16 eV respectively; ASCF + CI + Q values in Table 4) shows that this band can only be assigned to the first ionization of the aminomethyl radical CH,NH,. It is clear, however, from IR chemiluminescence studies of the F + CH,NH, and F + CH,ND, reactions [19] that both reactions 1 and 2 are rapid and, at room temperature, have rate constants which are ap- proximately equal and close to the collisional limit. Therefore both CH,NH, and CH,NH must be produced from the F + CH,NH, reaction, but the first band of CH,NH is not seen probably because of overlap with the much

YF

l-r

i

HCN

1

231

Cti2Nti;(~~,l ---- _

-CH2NH2 CXAI

I HetHe

x10

I ,

8 1 6 5 I.E.(eVl

Fig. 3. The first photoelectron band of CH,NH* recorded with HeI radiation. Abscissa: ionization energy (eV); ordinate: counts s-r.

more intense first band of methylamine. Also the second band of CH,NH, is expected in the 10.6-11.0 eV ionization energy region (see Table 4). This and higher bands of CH,NH, were not seen because of overlap with much more intense bands of CH,NH,, CH,NH and HCN.

Assignment of the vibrational structure in the first photoelectron band of CH,NH, can be readily achieved using the results shown in Tables 1-3. In making this assignment the following pieces of evidence should be borne in mind.

(a) Table 1 indicates that for the CH,NH,+( X A,) +- CH,NH,( X A) ionization the main changes in equilibrium geometry involve a decrease in the C-N bond length and a change of point group from C, to C,,. The C-N stretching mode in CH2NH2+( X A,) has been computed in this work at the

Fig. 2. He1 photoelectron spectra recorded for the reaction F + CH,NH, at constant reagent partial pressures. The mixing distances above the photon beam were (B), 2 cm; (C), 4 cm. (A) was recorded for the reagents F, and CH,NH,. Abscissa: ionization energy (eV); ordinate: counts s-i.

232

TZVP/MP2 level as 1788 cm-, in agreement with the experimental average separation of 1810 + 50 cm-. On this basis the main series in Fig. 3 is assigned to excitation of the C-N stretching mode in the ion, which is expected from the computed frequencies in Table 2 to have increased from the CH,NH,( X *A) value.

(b) The vibrational selection rules in photoelectron spectroscopy require that only those vibrational modes which are totally symmetric with respect to the common symmetry elements of the molecule and ion in their equi- librium geometries can undergo single quantum excitations in the ion. In Table 2 a vibrational mode of CH2NH2+( X A,) is computed at 1159 cm- at the TZVP/MP2 level and this corresponds to a CH, out-of-plane deformation. This transforms as b, in C,, symmetry and a in C, symmetry. This computed value compares favourably with the experimental value of 1190 + 60 cm- and, in view of this, the observed series is assigned to excitation of this mode in the ion. This assignment is reasonable in view of the expected change from a chair-like geometry to a planar geometry on ionization (i.e. (a) to (b) in Fig. 1).

(c) The computed vibrational frequencies of CH,NH+( X 3Ar) in Table 3, although calculated at the SCF level and expected to be about 10% too high [36,37], do not include values in the region 1600-2600 cm-. This would make the main series shown in Fig. 3 impossible to assign if the band shown in this figure were attributed to CH,NH. This is, therefore, a further piece of evidence in support of assignment of this band to CH,NH, and not CH,NH.

The first adiabatic ionization energy of CH,NH, measured in this work, 6.29 + 0.03 eV, compares favourably with 6.1 f 0.2 eV measured by electron impact mass spectrometry [3] and 5.86 eV computed in this work and in ref. [4] (see Table 4). If the heat of formation, AH&,,, of CH,NH,( X *A) is taken as 149 &- 8 kJ mall [35], then the experimental adiabatic ionization energy can be used to determine the heat of formation of CH2NH2+( X A,) as 755 f 11 kJ mol-, in reasonably good agreement with a previous determination of 744 + 8 kJ mol- obtained by electron impact mass spec- trometric studies on a number of alkylamines [46].

CH,NH, is isoelectronic with CH,OH and CH,F, and for these three CH,X molecules, the first ionization corresponds to removal of an electron from a molecular orbital which is 7r-antibonding in character in the C-X direction [6-81. As a result, the main vibrational structure in the photoelec- tron band consists of excitation of the C-X stretching mode in the ion with a frequency increased over that in the neutral molecule. Also, CH,NH, and C,H, are isoelectronic and show a number of similarities in their computed minimum energy geometries. C, H 4 - in its ground state is expected to adopt a chair conformation [47] similar to that of CH,NH,( X*A). Also, the

233

ground electronic state of C,H, is known to be planar [48], like CH2NH2+ (X Al), and the low-lying 3A state of C,H, has been computed to have a minimum energy geometry with the two CH, planes orthogonal, as in the geometry of CH,NH,+(a 3A) [49].

Although the first adiabatic and vertical ionization energies of CH,NH,( X 2A) have been measured directly for the first time in this work, it is unfortunate that the vibrational structure in the first photoelectron band has not been more clearly resolved. It may, however, be possible to measure the vibrational constants in CH,NH*+( X A,) more precisely by either recording the laser multiphoton ionization (MPI) photoelectron spectrum of CH,NH,( X 2A) or by recording its laser MPI mass-resolved ion-spectrum. In the former case, vibrational modes in the ion can be selectively excited by ionizing from Rydberg states of different vibronic character and in the latter case the observed vibrational structure will be that of a Rydberg state whose vibrational constants approximate to that of the ground state ion. Both experiments would be favoured as the first ionization energy of CH,NH, is very low (adiabatic value 6.29 + 0.03 eV) and to ionize the molecule would only require two photons at 394 nm. The analysis of such spectra would be assisted by the measurements made in this work and the computed vibra- tional frequencies shown in Table 2. It should be noted that mass-resolved ion-spectra have recently been recorded for CH,F [50] and CH,OH [51]. In these studies CH,F and CH,OH were ionized via (2 + 1) processes at 335-385 nm and 425-495 nm respectively.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the S.E.R.C. for financial support for this research and a postdoctoral fellowship (to E.P.F.L.). M.H.Z.N. also thanks the Iranian Government for support,

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