CH2N photoelectron

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    Photoelectron spectroscopy of CH2NDaniel C. Cowles , Michael J. Travers , Jennifer L. Frueh , and G. Barney Ellison

    Citation: The Journal of Chemical Physics 94 , 3517 (1991); doi: 10.1063/1.459773 View online: http://dx.doi.org/10.1063/1.459773 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/94/5?ver=pdfcov Published by the AIP Publishing

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    Photoelectron spectroscopy of NDaniel C Cowles, Michael J. Travers, Jennifer L Frueh, and G Barney EllisonDepartment of Chemistry and Biochemistry University of Colorado. Boulder Colorado 80309-0215

    (Received 29 June 1990; accepted 31 October 1990)

    We have measured the negative ion photoelectron spectra ofCH 2N - and CD 2N - and findthe electron affinities: EA CH 2N) = 0.511 0.008 eVand EA CD 2N) = 0.498 0.011 eV.Franck-Condon simulations of these spectra are carried out and we estimate the CH 2N - andCH 2 N geometry differences; we fit our spectra with the following [constrained] molecularstructures:

    H 1.106 A

    118 C = N/ [1.27 ]

    H

    We combine our EA CH 2N) with the results of previous gas phase ion studies to extract anumber of thermochemical parameters kcal/mol): Dg CH2N-H) = 85 5Dg H-CHN) = 23 6 Dg H 2C N) = 144 6 and the isomerization enthalpy ofH 2 CN + HCNH + is i l isom C v ..... C oov = - 51 7. Attempts to calculate the geometryand vibrational frequencies of the H2 CN radical are disappointing. Unrestri cted Hartree-Fockand second-order M 6ller-Plesset ab initio calculations in a 6-31 + + G** basis produce badlyspin-contaminated wave functions which do not reproduce the experimental findings.

    I INTRODUCTION

    We report a study of the photoelectron spectrum of thenegative ion isoelectronic with formaldehyde, CH 2N - .Photodetachment of CH 2 N - ion beams gives us the electron affinity of CH 2 N and allows us to examine the spectroscopy of this reactive radical. Most of the structure in ourphotoelectron spectrum arises from excitation of vibrationsand rotations in methylene amidogen, CH 2 N:

    CH 2 N - + Wo CH 2 N v,J) + e - KE). 1 )In addition to being interesting in its own right, the elec

    tron affinity is a link in many clever thermochemical cycles.Negative ion photoelectron spectroscopy permits a measurement of the radical electron affinity, EA R); gas phase ionchemistry uses proton transfer reaction kinetics to deter

    mine the hydrocarbon gas phase acidity,1 l H ~ c i dRR).

    These values may then be combined to extract the homolyticbond dissociation energy, D RR) :2,3

    D RR) = i l ~ c i dR-l l ) + EA R) - IP R) . 2)Use of Eq. 2) with established hydrocarbon acidities andelectron affinities provides one with a comprehensive pictureof the thermochemistry of I i system of related species. 4 Inthis case, we use EA CH 2 N) and the proton affinityS ofCH 2N - to extract D CH2N-H). This bond strength permits us to deduce both the heat of formation of the methylene amidogen radical and the C-H bond strength,D H-CHN).

    Methylene amidogen predominates in the reaction of Natoms with CH 3 radicals

    67 and is formed by the addition of

    H atoms to HCN.

    3)

    Methylene amidogen is also conjectured to be one of theprimary pyrolytic products 8 of the explosive, cyclic nitra

    mines, RDX and HMX:

    4)

    Loss of one or two hydrogens from the intermediate CH 2Nradical leads to the stable species, HCN and CN.

    The CH 2N radical was detected some time ago by EPRspectroscopy9 among the photolysis products of HCN isolated in an argon matrix. EPR techniques were later used toobserve CH 2 N in an assortment of r radiolysis studies 10 andin a discharge of protons and solid HCN. I These magneticresonance spectra provided conclusive evidence for equivalent protons in CH 2N and established the radical as a C vspecies. There is an important paper lO which discusses theEPR spectra of CH 2 N and the unusually large hyperfinecoupling constants for the H atoms, roughly 87 G. Hyperfine constants of this magnitude imply that each proton isstrongly coupled to the radical center on N and indicate anunusually short C = N bond and a powerful, hyperconjugative interaction of he in-plane a-like radical with each of the

    electron pairs of the C-H bonds. An in-plane a radical 10-cated on the nitrogen atom is certainly consistent with all ofthe EPR studies that have been published: 9-11

    J. Chern. Phys. 94 5), 1 March 1991 0021-9606/91/053517-12 03.00 1991 American Institute of Physics 3517article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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    3518 Cowles et al : Photoelectron spectroscopy of CH 2N-

    (5)We anticipate 12 a photodetachment transition from a

    closed shell CH 2N - ion to aCT type CH 2N radical as depicted in Eq. 5).This process is analogous to the photoionization of formaldehyde.13

    Ion-molecule chemistry studied in a flowing afterglowdevice and an ICR has contributed much to our understanding of the thermochemistry of the CH 2 N system. In a survey14 of the ion chemistry of CH 2N - , t l l l ~ i dCH2N-H)was reported as 388 5 kcallmol while the EA CH 2N)was bracketed around 0.5 0.1 eV. Preliminary photodetachment studies 15 of the CH 2 N - ion suggested an

    EA CH 2N)0 . 5 2

    0.05 eV. In an ICR spectrometer16

    the gas phase chemistry of HC==NH + was used to fix theheat offormation of methylene mine. By detecting the positive ions, the following reactions were monitored:

    HC==NH+ 2-methylpropane-+no reaction 2-methylbutane-+no reaction

    The neutral product resulting from hydride transfer toHC==NH + reaction is undoubtedly CH 2NH. Based on thehydride affinities of2-methyl butane and methylcyclohexaneit was possible to bracket the heat off rmation of the imine,tlllJ298 CH 2 NH) = 26.4 3.2 kcallmol.

    Recently the ionization potential of CH 2N has beenmeasured 17 in a series of electron impact and synchrotronthreshold experiments; IP CH 2N) = 9.4 0.1 eV andIP CD 2N) = 9.4 0.1 eV. These are important measurements since they allow us to calculate the heat of formationof the C v cation, CH 2N + .

    CH 2N is the subject of a number of recent ab initio calculations. The structures of the CH 2 N - ion and CH 2N radical are predicted by a Hartree-Fock calculation in a 631 G** basis; second-order M011er-Plesset correlationcorrections to the HF energy are used to estimateEA CH 2N) aofO.3 eV.

    18 In a separate study, GVB-CI techniques are used to scrutinize the potential energy surface forreaction 3) and the relative product energies and reactionbarriers are calculated. 19

    Most recently, an infrared absorption spectrum of matrix-isolated CH 2N was obtained and most of he vibrationalmodes identified. 2 Several of the properties of methylene

    amidogen are collected in Table I. An extensive review ofthemolecular properties of the CH 2N radical has recently beenpublished. 21

    II. EXPERIMENT L

    The instrumentation we use to carry out these experiments has been described in some detail elsewhere. 22 .23

    Briefly, ions are prepared in a conventional dc electric discharge at a pressure of approximately 0.1 Torr. The CH 2 Nion is produced in a discharge of methyl azide CH 3N 3 ) andN 20 where a 0.015 in. diam. tungsten wire filament serves asthe cathode; weak ion beams of the deuterated ion, CD

    2N - ,

    could be produced from a discharge ofCD 3N 3, NH 3 , andN 2. Note that a different preparation scheme for CD 2 N -

    (6)

    was necessary due to interference from NO - . Sufficientquantities of 0 - and NH 2 ions are created in the respectiveplasmas for calibration purposes.

    Methyl azide was prepared by the method of Livingstonand Ra0

    24 without significant deviation; purification wasaccomplished by two freeze-pump-t haw cycles followed byvacuum distillation. Since methyl azide is highly explosiveonly small amounts ( < 5 g) were prepared as needed.

    Negative ions are extracted from the plasma, focusedand accelerated to form a 600 eV beam which is then massanalyzed by a Wien velocity filter and delivered to the highvacuum chamber 10- 9 Torr) for detachment. Typicalmass-selected beam currents for CH 2N - and CD 2N - are300 and 75 pA, respectively.

    The mass-selected ion beam is intersected by the intracavity radiation of an Ar ion laser mounted such that theplane of polarization of the l ight beam is fixed at the magicangle (54.7) with respect to the electron collection direction. Our laser is operating CW on a single line w o = 488nm or 2.540 eV) with approximately 75 W of circulating

    TABLE I. Established properties for the methylene amidogen radical.

    A. Thermochemistry

    ~ H ~ d ( C H 2 N H )

    ~ J 2 9 8 ( C H 2 N H )

    IP CH,N)

    IP CD,N)

    388 5 kcallmol26.4 3.2 kcallmol9.4 0.1 eV9.4O.1 eV

    B. CH, N Matrix) vibrational frequencies cm - , )Mode Symmetry CH, N

    V CH, sym. stretch) aV2 CN stretch) a 1725.4V3 CH 2 scissors) a 1336.6v. CH, N umbrella) b 954.1Vs CH

    2 asym.stretch)

    b2 3103.2v. CH, rock) b 912.8

    Reference

    14161717

    Ref. 20CD,N

    1073.3776

    2427.S?

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    Cowles et 81 : Photoelectron spectroscopy of CH 2N - 3519

    power. A small portion of the detached electrons 10 - 3) iscollected and energy analyzed by a pair of electrostatic hemispheres arranged in series. Spatially dispersed electrons exitthe hemispheres, strike a position sensitive array detectorand are counted. With the aforementioned ion beam currents we analyze 10-100 counts/s with an operating resolution of approximately 20 meV FWHM).

    The photoelectron spect rum must be transformed to thecenter of mass CM) reference frame and the energy scalecalibrated with a reference ion. To calculate the CM kineticenergy ofthe detached electrons we use the following expression:

    KE = KEcal + r V al -- V lit ) + m W [2__ _].M Meal

    7)

    In this expression, KE is the CM kinetic energy of the electron detached from the ion of mass M V lit is the voltagethrough which the electron must be accelerated to reach the

    transmission energy of he analyzer V. lit =o

    - KE). Thekinetic energy scale is fixed by detaching a calibration ionKE eal = lUu o - EA eal ) of mass Meal with a well-known

    electron affinity. We use [EA O) = 1.461110 0.000 001 eV]ZS and N H [EA NH 2 ) = 0.771 0.005

    800CH2N

    fZ = 28Laser 1 0 = 488 nm 2.540 e V)

    CIl

    600

    0U

    Z0

    t3 400C

    r t:t0f o0::t:rl.

    200

    D

    o

    eV]Z6 as calibration ions since they are produced simultaneously with CH 2 N - and CD 2 N - , respectively. Veal is thevoltage needed to allow transmission of the calibration ionelectrons; m is the mass of an electron; W is the beam energyin eV; and r is a dimensionless energy scale compressionfactor typically found to be 1.000 0.005).21 By varyingthe value of V lit we are able to sample electrons with different kinetic energies.

    III. RESULTS

    The photoelectron spectra of H z N - a nd z N - areshown in Figs. 1 and 2; Table II is a listing of the CM kineticenergies of he observed transitions for each of he features inFigs. 1 and 2 We will argue in the discussion section thatfeature A is the 0,0) band of the photoelectron spectrum;hence this peak furnishes us with the raw electron affinity.Some corrections must be considered before it is possible toextract an electron affinity from these data. We must account for the unresolved rotational transitions which are

    blended together in our spectra. In addition, we must payattention to any corrections associated with possible spinorbi t effects and the compression of our energy scale .

    The first correction is due to the unresolved rotationalmanifold under each peak. For an asymmetric rotor the

    A

    B

    1.2 1.4 1.6 1.8 2.0 2.2 2.4

    CM PHOTOELECTRON KINETIC ENERGY eV)

    FIG. 1 Photoelectron spectrum of CH 2 N - . Data point spacing is roughly 5 meV

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    3520 Cowles et al.: Photoelectron spectroscopy of CH 2 N

    150A

    MfZ = 30Laser 1..0= 488 nm (2.540 e V

    100

    B

    c

    50

    o

    1.2 1.4 1.6 1.8 2.0 2.2 2.4

    CM PHOTOELECTRON KINETIC ENERGY (eV)

    FIG. 2. Photoelectron spectrum of CO 2 N - . Data point spacing is roughly 5 meV.

    band centroid is shifted from the rotational origin by anamount 28

    where the rotational constants A B and C ' belong to theradical while A , B , and C are those of the ion) must beapproximated. We estimate the rotational temperature ofthe ions to be T= rot r;;5, TVib = 800 200 K.:l k T [

    A ' H ' C 3] ( B - B ' )rot B 2A 2H + 2C 2 + 3 '

    The second correction attempts to account for the effects of spin-orbit levels on the determination of electron8)

    TABLE II. CM Photoelectron kinetic energies (eV).

    Laser o = 488 nm (2.540 eV).CH 2 N CD 2 N Assignment

    a 2.200 0.050 2.160 0.060 31370cm -I 950 em-I

    A 2.030 0.006 2.042 0.009 (0,0),2:,3:1330 em -I 96Oem- 1

    B 1.865 0.006 1.923 0.010 3 ~ 2 ~ 3;1480cm- 1080em--

    C 1.681 0.006 1.789 0.010 n 3 ~ 1 ~ 3 :1330em- 1 101Oem- 1

    D 1.516 0.009 1.664 0.015 1 ~ 3 ~ 1 ~ 3 l1450em- 101Oem- 1

    E 1.336 0.015 1.539 0.020 1 ~ 1 ~ 3 ~1260 em -I

    F 1.180 0.020 1 ~ 3 ~

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    Cowles e t at.: Photoelectron spectroscopy of CH 2N - 3521

    affinities. In the CH 2N

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    3522 Cowles 9 a/ : Photoelectron spectroscopy of CH 2N -

    800 -

    400 -

    200 -

    CH2N

    MfZ = 28Laser '0 = 488 nm 2.540 e VFranck-Condon simulationusing v2 with Tvib = 800 K

    D

    C

    B

    I

    A

    F

    E : ~ I: 1 . . 1 a~ y -o - r ~ ~ ~ ~ ~ ~ ~ - ~ ~ ~ ~ ~ ~ ~ = = = = = = ~ = = = = = = ~ ~ ~ = =

    I I I I I I I1.2 1.4 1.6 1.8 2.0 2.2 2.4

    CM PHOTOELECTRON KINETIC ENERGY eV)

    FIG. 3. Franck-Condon simulation of the photoelectron spectrum of CH 2 N - using only the C = N stretching mode 2 ) .

    potentials for both the ion and the neutral; hence the vibrational functions for these modes, t lf (Q2) and t v(Q2) areeigenfunctions of a l inear oscillator:

    H V i b Q 2 ) = ~ 2 P i+-l-wiQi.~

    10)

    In expression 10), P2 and Q2 are the momentum operatorand the C N stre tch coordinate, W 2 is the C N harmonicfrequency and ti is the effective inverse mass, expressed asan elementl of the matrix. It seems evident from Fig. 3that the C N stretch mode is nearly inactive and contributes very little to the photodetachment spectrum ofCH 2 N - . The C = N bond length of he CH 2 N - ion must bevery close to that ofthe CH 2 N radical. 31

    In Fig. 4, we simulate the data using all the symmetrica l ) modes V I v2 and V 3 As with V 2 , the scissoring mode

    (v 3 ) is also well rep resented by a simple harmonic oscillator.However, since we have extracted a value for the C-H bonddissociation energy ofCH 2 N vida infra) that is unusuallylow, Dg H-CHN) = 23 6 kcallmol, the C-H oscillatorslive in a shallow well. Therefo re we must use an anharmonicpotential to accurately model the symmetric C-H stretchingmode (VI ) t is convenient to represent this mode in boththe radical and the ion with a Morse oscillator where the

    frequencies W t l = 2860 cm I and xii = 20 cm I providethe best fit to our data:

    11 )

    The fit solid curve) to our experimental spectrum points)in Fig. 4 is quite reasonable.

    Within the Bom-Oppenheimer approximation, thesame geometry that is used to fit the CH 2 N - spectrum mustalso fit the CD 2 N - spectrum. A very important test of ourmodel is that it properly accoun t for all of he CD 2 N - isotopic shifts observed in Fig. 2. The CD

    2N frequencies must be

    related to those ofCH 2 N by the matrix elements appropriate for VI , V 2 and V 3 The predicted Franck-Condon profileis displayed with th e CD 2 N - data in Fig. 5. We believe theresults of our model are quite reasonable since all of the frequency shifts are faithfully reproduced, however the intensities of the peaks are not exact. I t shoul d be emphasized thatour CD 2 N - experimental spectrum is at a very low S / N Wewere never able to produce intense ion beams only 75 pAlof CD 2 N - from CD3N3 although our CH 2 N - currents

    300 pA) were quite acceptable.Since we can fit both the CH 2 N - and the CD 2 N - spec

    tra with one model, we believe that our simple Franck-Condon picture based on three uncoupled oscillators is a plausible representation for our photodetachment spectra. The

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    Cowles et al : Photoelectron spectroscopy of CH 2N- 3523

    800

    fIl

    600

    0t ,)

    Z0

    t 400~Il :/

    0

    b:I:ll.t

    200

    MIZ = 28Laser 1..:: 488 nm (2.540 eV)

    Franck-Condon simulationusing VI. v2. and v3 withT vib = 800 Ji(

    (0,0)

    1.2 1.4 1.6 1.8 2.0 2.2 2.4

    CM PHOTOELECTRON KINETIC ENERGY (eV)

    FIG. 4. Franck-Condon simulation of the photoelectron spectrum of CH 2 N - using all of the totally symmetric modes: V I V 2 and V 3

    final geometrie s and frequencies chosen are summarized inTable III.

    B Ab Initio calculations

    A series of ab initio calculations was carried out on theCH 2 N radical and CH 2 N - anion to gain insight into theelectronic structures of these species and so to illuminate ourexperimental findings. We use the GAUSSIAN 86 package ofcomputational programs 32 with a standard Gaussian basis

    set augmented with polarization and diffuse functions, the 6-31 G** basis.

    Several ab initio studies of the CH 2 N radical19

    33 and

    one of the anion IS have been reported. In a detailed analysisof he reaction pathways for H HCN, Bair and Dunning l9

    determined the electronic structure of the CH 2 N adduct andthe corresponding transition state. These studies were basedon GVB-CI wave functions calculated in a large Gaussianbasis set. Equilibrium geometries and harmonic vibrationalfrequencies were reported for CH 2 N and its isomers. TheirCH 2 N GVB-CI geometry was verified by Jacox

    20 in her normal coordinate study and adopted by us in our Franck-Condon analysis of the photodetachment of CH 2 N - .

    We treated the ion with a Hartree-Fock (RHF) wavefunction in a 6-31 G basis set and calculated the har-

    monic frequencies at the predicted equilibrium geometry.Our findings reproduce the results reported earlier. IS OurRHF calculations approximate the anion wave function as asimple antisymmetrized product of doubly occupied orbitals. In the expression below the antisymmetrizer or determinant operator is written as d

    l e A I = d{lai2aiJa i4ai Sai Ib b Ib n (12)

    The calculated ion geometry and frequencies are summarized in Table IV.

    For the CH 2 N radical, a Hartree-Fock (UHF)6-31 G** calculation was initially performed and gavesurprising results (see Table IV). Notice that the UHF calculation predicts a substantially longer C N bond lengthfor CH 2 N than was determined for CH 2 N - in the RHFcomputation, an obvious contradiction of the experimentalphotodetachment spectra (Figs. 1 and 2) which clearlyshow no excitation of the C = N stretch, v2 Furthermore,when we compare the UHF {i) S to the experimental frequencies of Table I, a curiously low value for {i)2 is observed.Specifically, it is a common finding 34 that when comparedwith experiment, the harmonic frequencies of HF calculations are too high by about 10 . This trend is observed in allthe calculated frequencies save {i)2 the C N stretch .Whereas {i 3 through {i)6 are too large by 6 to 16 , the

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    3524 Cowles t al: Photoelectron spectroscopy of CH 2N-

    150

    100

    50

    o

    CDZN

    MIZ = 30Laser A.. = 488 run (2.540 eV)

    Franck-Condon simulationusing vI. vz. and v3 withTYib = 800 K

    (0.0)

    I ...... v ~ .J. ..... .

    1.2 1.4 1.6 1.8 2.0 2.2 2.4

    CM PHOTOELECTRON KINETIC ENERGY eV)

    FIG. 5. Franck-Condon simulation of the photoelectron spectrum of CO 2 N - using all of the totally symmetri c modes: V I V2 and v,.

    UHF value of Ct 2 (1631 cm - 1) is actually lower than theexperimental value 20 of 1725.4 cm - 1 (by 6 ). We expectedthe UHF Ct 2 to be about 1900 cm - 1.

    We believe that these anomalous results stem from aspin-contamination problem inherent to the UHF method. 35 Since the a and 3 orbitals are permitted to vary independently in a UHF calculation, it is well known that theypolarize each other. 36 We find that our UHF vectors forCH 2N are badly spin-contaminated with S2)UHF equal to0.981 rather than the exact value of l Our UHF wave function (which should be a doublet) evidently mixes stronglywith a quartet state. The pair of states to consider is depictedbelow:

    H*H ~H ~(13)

    The essential feature of he 2 11 B 2 ) state is that it is a singletcoupled 1T electron pair (1T(1T, ) combined with the u orbital

    b 2 ) electron as an overall doublet. This state might be represented as follows:

    1 11(B 2 = . i f{ a ~2 a ~3 a ~ 4 a ~5 a ~Ib nx 1T 1T,u) a /3- /3a)a} . (14)

    The UHF method mixes this 12 11 B 2 doublet state withthe 14 11 B 2 ) ) quartet state in which the ( 1T(1T, ) electron pairis now triplet coupled and joined with the u b 2 electron.

    14 11 B 2 = . i f [ ~ 2 a ~3 a ~ 4 a ~ 5 a ~Ib ]x 1T 1T,u) a /3+/3a)a} . (15)

    I t seems plausible that the UHF method might blend some ofthe character of the 14 11 B 2 ) ) state into the 12 11 B 2 ) ) function. Expression (13) suggests that such a mixing wouldlengthen the C N bond and lower the vibrational frequency, as was found (Table IV).

    To explore a possible connection between S2) and theharmonic frequencies, we then carried ou t a restricted, openshell Hartree-Fock (ROHF) 6 31 + + G** calculation onthe CH 2 N radical. This computation constrains all of thedoubly occupied a and 3 orbitals to be identical and forcesS2) to be the required i. Naturally these additional restric

    tions raise the energy of 11 ROHF above that of 11 U H F TheROHF calculated structure for the CH 2 N radical differs

    with the UHF geometry (see Table IV) and is about 10mhartree higher in total energy. The C N bond length

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    Cowles e/ al.: Photoelectron spectroscopy of CH 2N -

    TABLE IV. Ab initio calculations in a 6-31 + + G** basis.

    HZ082 A

    119 C N

    / 1 2 3 9 AH

    ~ 4 A_109 C N

    1.240 A

    XeB,) XeB,) X( A,)E(UHF) = - 93.439 558 hartree E(ROHF) = - 93.429 299 hartree E(RHF) = - 93.395 863 hartree

    A = 9.628 cm - ,B = 1.287 cm - ,C = 1.135cm- 1

    Ji.o = 2.52 Debye

    H ~ 9 1A117 C N

    / 1 2 2 3 AH _

    x e B , )

    A = 9.642 cm - ,B = 1.328 cm - ,C = 1.167cm- 1

    Ji.o = 2.64 Debye

    A = 9.955 cm - ,B = 1.279 cm - ,C = 1.133 cm - I

    ~ 9 A_108 C N

    1.256 A

    X( A , )E(UMP2) = - 93.701783 hartre.e

    A = 9.614cm- 1E(RMP2) = - 93.720 564 hartree

    A=9.872cm- 1

    B= 1.352cm-1

    C = 1.185 e m -Ji.o = 2.54 Debye

    Mode

    (z), (CH, sym. stretch) a ,(z) , ( C = N stretch) a ,(z)3 (CH, scissors) a(z) . (CH, umbrella) b(z )s (CH, asym. str. ) b(z) . (CH, rock) b

    Harmonic vibrational modes cm - )CH,NXeB,)

    B = 1.240 cm - ,C = 1.102cm-

    (UHF) (ROHF) UMP2 ) GVB-CI) Ref. 19) (RHF) (RMP2)

    3210 3238 3118 3031 2772 25941631 1896 2080 1722 1765 16991431 1526 1454 1407 1623 15441041 1152 1170 1012 1089 10583299 3328 3199 3098 2600 23321066 1080 976 1007 1265 1171

    TABLE V. Thermochemical values kcal mol-I).

    Quantity

    EA(CH,N)

    EA(CD,N)

    IP(CH,N)IP(CD,N)

    All ;,, d (CH, N-H)

    aHJ(CH,NH)a H ~ C H 2 N )

    aHJ(CH,N - )aHJ(CH,N+ )aHJ(HCNH + )

    D(CH,N-H)D(H-HCN)

    D(H,C=N)D(H,C=NH)D(H,C=O)

    OK

    11.8 0.211.5 0.2

    386 5

    28.3 3.261 650 6

    228 3

    85 523 6

    144 6ISO 3177.8 0.6

    298K Derivation

    Electron affinities ionization potentials11.8 0.2 photodetachment spectroscopy

    photodetachment spectroscopy217 2 photoionization spectroscopy217 2 photoionization spectroscopy

    Gas phase acidity388 5 proton transfer kinetics

    Heats of formation26.4 3.2 hydride transfer kinetics

    6 0 6 D(CH,N-H) - aH/(H) + aH/(CH,NH)4 9 6 aH/(CH,N) - EA(CH,N)

    277 6 IP(CH,N) + All/ (CH,N)227 3 aH/(HCN) + aH/(H + ) - PA(HCN)

    Bond dissociation energies86 5 aH,dd (CH,N-H) + EA(CH,N)-IP(H)24 6 All/(HCN) + aH/(H) - aH/(CH,N)

    145 6 All/(CH,) + All/(N) - aH/(CH, N)152 3 aH/(CH,) + All/(NH) - aH/(CH,NH)

    179.4 0.6 aH/(CH,) + All/(O) - aH/(CH,O)Isomerization energy

    - 51 7 aH/(HCNH+) - aH/(H,CN+)Auxiliary values Ref. 48)

    3525

    Reference

    This workThis work

    17

    17

    14

    16

    PA 300 (HCN) = 171.42 Ref. 49); aHJo(CH,) =92 .80 .6 Ref. 50); aHJo(NH) = 85.40.3 Ref. 51); aHJo(CH,O) = - 2 6 . 0 0 . 1 Ref.52); aHJo (HCN) = 32.4 2 A l l ~ o(H) = 51.6336 0.0014; aHJo 0 ) = 58.984 0.024; a H ~ o(N) = 112.53 0.024; aHJo H + )= 365.221 0.009, aHJo (H - ) = 34.242 0.005

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    3526 Cowles fiJt a/ : Photoelectron spectroscopy of CH 2N -

    (1.239 A is now nearly identical to that of the RHF ion(1.240 A), in good agreement with our experimental findings which indicate that oRcN ,0 .02 A. Several of theROHF {w} values are similar to the UHF {w} results exceptfor W which is now 1896 cm - I. In summary, we believe thatthe ROHF calculation bet ter represents the CH 2 N radicalsince the computed harmonic frequencies are roughly 10%higher than the experimental values and the C N bondlengths are comparable in the ion and neutral.

    The contribution of electron correlation to the potentialenergy surface is certain to be large. We attempt to quantifythis important effect via perturbation methods by application of the second-order MQSller-Plesset correction, (MP2).Table IV shows that when we reoptimize the CH 2 N - RHFgeometry, the C-H bond lengthens to 1.139 Aand the C Nbond grows to 1.256 A; the total energy of the ion falls by0.325 hartree, about 8.8 eV This post Hartree-Fock treatment also leads to important changes in methylene amidogen; reoptimization of the UHF CH 2 N geometry leads to aserious shortening of the C N bond length from 1.260 to

    1.223 A n the UMP2 radical. The energy of he fully relaxedUMP2 CH 2 N radical falls by 0.262 hart ree (7.1 e V). At thislevel of computation, CH 2 N - is now bound with respect tothe CH 2 N radical by 18. 78 mhartree (0.5 e V or 12 kcallmol). The contraction of the C N bond in the UMP2 radical is attended by a dramatic rise in the vibrational frequencyas W goes from 1631 to 2080 cm - I. The charge distributionin CH 2 N does not seem to vary much; the dipole moment,f lD only changes slightly (2.52 D -- 2.54 D as one incorporates correlation.

    We are disappointed with the results of these standard ab initio computations. The vibrational frequencies

    tabulated in Table IV are only an imperfect match to theexperimental findings of Tables I and III. The unrestrictedHartree-Fock method necessarily produces a spin-contaminated wave function which leads to an incorrect C N bondlength and a faulty C = N vibrational frequency; both RCNand W are too small. All attempts' to remedy this UHF failure by perturbation methods (Mf i ) were unsuccessful. Oneshould use a projected MP2 method with full geometry optimization 37 to attack this problem but our set of routine computer codes does not include this option. Likewise we couldnot pursue a MQSller-Plesset correction to the ROHF vectors. We conclude that the radical geometry and harmonicfrequencies from the GVB-CI method are superior to ourHartree-Fock and second-order MQSller-Plesset results.

    c hermochemistry

    We can make use of the electron affinity of CH 2 Nandits recently measured ionization potential to extract someimportant thermochemical values; these findings are collected together in Table V. I t is interesting to note that CH 2 N isisoelectronic with vinyl4 radical, CH 2 CH, and that the electron affinities are comparable:

    H , AV 0.511 0.008 eVC = = N

    H / (16)

    0.667 0.024 eV

    The ionization process for the CH 2 Nand CH 2 CH radicalswill be more complicated since the resulting cations rearrange extensively. Photoionization 38 of vinyl radical leads to

    a bridged39

    cation, C 2 H3+; the observed spectrum is verycomplicated with IP(CH 2 CH) = 8.59 0.03 eV whichcontrasts with that of methylene amidogen,IP(CH 2N) = 9.4 0.1 eV.

    Use of Eq. (2) enables one to find the N-H bondstrength of methyleneimine as g (CH2N-H) = 85 5kcallmol.

    8S S

    XIA X B (18)

    The heat of formation of CH 2NH has been established fromthe bracketing studies described in Eq. (6). Therefore use ofthe dissociation energy from Eq. (18) yields the heat of formation of the radical CH 2N. Then we can use the heat offormation of HCN to extract the C-H bond strength ofmethylene amidogen, g (H-CHN) = 23 6 kcallmol.

    H",/ C = = N .... H - C = N

    H X2B2 X I l ;

    23 6

    +H. (19)

    This very weak C-H bond is reminiscent of that in formylradical, H-CO, and the vinyl radical /3 C-H bond. In theearlier GVB-CI s tud y l9 of the addition ofH atom to HCN,

    g(H-CHN)

    was estimated to be19

    kcallmol, a valuecompatible with Eq. (19).Prior to deriving the double-bond energies for CH 2==0,

    CH 2= N H and CH 2= N in Table V, we guessed that quantum mechanical exchange stabilization might be manifested 40 in these values. Specifically, rupture of the doublebond generates CH 2 in each case but CH 2N also produces(4S)N atoms.

    144 6

    (20)

    Thus, while the dissociation energies for the isoelectronicspecies CH 2==0 and CH

    2= N H should be comparable,

    the exchange stabilization associated with the high spinN atoms should lead to a lower dissociation energy forCH 2= N . We find

    4 g (C H 2==O) = 177.8 0.6 kcal/mol, g (C H 2= N H ) = 150 3 kcal/mol and

    g (C H 2- N = 144 6 kcal/mol. Admittedly, morestriking examples of exchange stabilization have been documented. 42

    Finally we can use our heat of formation for CH 2N withthe recent measurement ofIP (C H 2N) to glean some insightinto the rearrangement energetics of the isomeric cations,CH 2 N+ and HCNH+. From Table V, :JIJ298 (C H 2N+)is 277 6 kcallmol while proton affinity studies of HCNhave fixed :JIJ298 (HCNH +) as 227 3 kcallmol. Thuswe have an estimate for the isomerization energy of

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    Cowles et al.: Photoelectron spectroscopy of CH 2N - 3527

    21)

    The linear HCNH + ion is thought43

    to be a major determinant of interstellar [HCN]/[HNC] ratios and has been deected in the laboratory44 and in interstellar clouds. 45 Theoetical studies predict 46 that the C 2v isomer, CH 2 N + , is

    unstable with respect to isomerization to the linear ion. Acareful search of the energy gradients about the C 2v geomery reveals no barrier between CH 2 N + and HCNH + . The

    most recent estimate 47 of the energy difference between theC zv and C oov structures is 74 kcallmol. However, note thatynchrotron experiments which ionize CH 2 N produce a

    CH 2 N + ion with the geometry of the radical so that ourestimate of 51 kcallmol can only be loosely compared to

    hese ab initio results.CKNOWLEDGMENTS

    We would like to thank Dr. R. Bruce Klemm BNL)and Dr. Louis J. Stief NASA) for preliminary communicaion of the CH 2 N ionization potential. We are also indebtedo Dr. Bill Kirchoff DOE Chemical Physics) for alerting uso the importance of these synchrotron measurements of the

    CH 2 N IP. We have also benefited from discussions aboutpin-contaminated UHF and UMP2 wave functions with

    Dr. Mark S. Gordon NSF). This work was supported by agrant from the Chemical Physics Program, United States

    Departmentof

    Energy DE-FG02-87ERI3695). The elecronic structure and Franck--Condon calculations were caried out on a J l VAX acquired with the help of the National

    Science Foundation CHE-8407084).

    c. R. Moylan and J. I. Brauman, Annu. Rev. Phys. Chern. 34, 1871983).

    Ionization potentials and electron affinities are adiabatic enthalpychanges strictly defined at 0 K, while I 1 H ~ i dand D ~ 9 8commonly represent changes of enthalpy at 298 K. Equation 2) is more properly writtenas

    D ~ 9 8R-H) = I 1 H ~ dR-H) + EA(R) - IP H) - < I 1 C p ~ 9 8 .The difference in the integrated heat capacities can be written in the fol-lowing manner: < I 1 C p ) ~ ' 8 = = < C ~ ( 1 ) [ R- 1 - C ~ 1 ) [ Rm8C ~ 1) [H + 1 - C ~ 1 ) [H > ~ 9 8 .In the absence of low-lying electronic states, the heat capacity correction is generally less than 0.5 kcallmol since the structures and vibrational frequencies of the ions and neutrals are similar. Because the unC ,rtainties of the I 1 H ~ dmeasurementsare commonly 2 kcallmol or greater, this correction is usually ignored;thus I 1 C p ) ~ 9 ... 0 kcal lmol. See Ref. 3, Sec. 3.3.) We adopt theconvention that Cp (e - =0. In Table V we have explicitly applied the heat capacity correction to obtain proper Dg R-H) and D ~ 9 .R-H) values.S. G. Lias, J. E. Bartmess, J. F. Lil:bman, J. L. Holmes, R. D. Levin, andW. G. Mallard, Gas-Phase Ion al/d Neutral Thermochemistry, J. Phys.Chern. Reference Data 17, Supplement No. I 1988).K. M. Ervin, S. Gronert, S. E. Barlow, M. K. Gilles, A. G. Harrison, V.M. Bierbaum, C. H. DePuy, W. C. Lineberger, and G. B. Ellison, J. Am.Chern. Soc. 112, 5750 1990).The proton affinity of the ion is identical to the gas phase acidity of thec o r r ~ p o n d i n gneutral; thus PA CH 2N -) ~ t r . c l dCH 2N-H).

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    tungsten atom. De tachment of he W - ion produces several states ofW Ijone can detect W[ a 5D J = 0 ) J - W- , W[a D(J= I ) J - W- ,W [ a D ( J = 2 ) J - W- , W[ a 5D J = 3 ) J _ W- , W[a 6D ( J = 4 ) J- W-, as well as W[ a 7S(J = 3) J - W-. These intervals have beenestablished by atomic spectroscopy and are tabulated by Charlotte E. Moore,Atomi c Energy Levels, Vol. II, NSRDS-NBS 35 US GPO, Washington,DC, 1971). Use of these precisely known intervals fixes y

    28 P. C. Engelking, J. Phys. Chern. 90, 4544 1986).29S. V. ONeil and W. P. Reinhardt, J. Chern. Phys. 69, 2126 1978 ).30E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, Molecular Vibrations

    McGraw Hill, New York, 1955). Appendix VI, pp. 303-306. We use thefollowing expressions in which 9 is the H-C-H angle and the C-H bondlength is r; the reciprocal masses for C and H are written asf lH == mass hydrogen) - I and f lc == mass carbon) - I; til = /lH

    lc 1 + cos 9) , ~ 2 = /lc + IN ti3 = 2 lH r + 2 lc lr l - c o s 9 ) .

    31 From our modeling of the CH 2N - and CD 2N - spectra, we believe thatORCN cannot exceed 0.02 A Consequently RCN CH 2N) = 1.268 Aimplies that RCN CH 2N - ) = 1.27 0.02 A

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    J. Chern. Phys., Vol. 94, No.5, 1 March 1991article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.72.2.27 On: Wed, 06 Nov 2013 05:07:31

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    3528 Cowles et a/ : Photoelectron spectroscopy of CH 2N

    41 It is also pertinent to recall that g (C H 2 = C H 2 ) = 171.0 1.2 kcaImol; see Ref. 4

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