J. Phys. Chem. 1995,99, 1649-1654
1649
He I Photoelectron, Photoionization Mass Spectroscopy, Mid-Infrared, and ab Znitio Study of the Unstable CH30CN Molecule Tibor Pasinszkit and Nicholas P. C. Westwood* Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada, NIG 2WI Received: August 9, 1994; In Final Form: October 5, 1994@ Pure gaseous methyl cyanate, CH30CN, has been generated from the gadsolid reaction of 0-methyl thiocarbamate with mercuric oxide; excess water is removed in-situ with P2O5. Methyl cyanate reacts instantly with metal salts and isomerizes readily to the isocyanate. The gaseous molecule is characterized by He I photoelectron spectroscopy, photoionization (He I, HLa,g,y) mass spectroscopy, and infrared spectroscopy for the first time. The geometries of the ground state molecule and cation, as well as the 10 lowest excited ionic states, are obtained from ab initio calculations at the MP2/6-31G** and CIS/6-31G** levels of theory. The observed ionization potentials and vibrational frequencies are compared with the results of the ab initio calculations, supporting the identification and assisting with the assignments.
Introduction The NCO group, like NCS, is a unique substituent because ligands can bind at either end. The isocyanates (R-NCO) are well-known, having been extensively studied by both experimental and theoretical methods. Very little, however, is known about the isomeric cyanate species (R-OCN), which are anticipated to be very different in their chemical behavior, structures, and spectroscopy. For spectroscopic investigation the parent cyanic acid (HOCN) and its methyl- and halogen-substituted derivatives would provide the most interesting challenges. Both HOCN and CH3OCN are strong candidates for interstellar molecules, and thus their generation and investigation have important astrophysical implications. The parent acid, HOCN, is expected to be more unstable than the methyl derivative and has not been identified in the gas phase, although it has been shown by infrared (IR) spectroscopy to arise from irradiation of matrix-isolated isocyanic' or fulminic acids2q3 A synthesis of the methyl derivative, CH30CN, was reported in 1965,435although a pure sample could not be obtained for spectroscopic investigation due to the instability of the molecule; undiluted methyl cyanate is so labile that it cannot be kept at room temperature, even for a few minutes, without decomposing, trimerizing, or isomerizing to the more stable isocyanate i ~ o m e r . The ~ IR spectrum was, however, recorded in CC4 s ~ l u t i o n . ~The , ~ generation of gaseous CH30CN (contaminated with water) for microwave investigation was recently reported,Eyielding an r, structure of C, symmetry with a C-0-C angle of 113.8'. This remains the only gas-phase study of this molecule. Nothing is known experimentally about the halogen cyanates, XOCN. Due to the paucity of detailed experimental work, theoretical ab initio calculations become important in providing information on structures and relative energies of such molecules. The structure and isomerization of HOCN has been the subject of detailed theoretical i n v e s t i g a t i ~ n , ~ as ~ ~have - l ~ the equilibrium structures of CH30CN,11,'2,14 FOCN, and C10CN.11-'2 The calculations show a bent and rigid frame for all these molecules and indicate that CH3OCN is some 19.5 (4-31G) to 27.1 (MP2/ 6-31G**) kcal mol-' less stable than CH3NCO. Also of relevance to this work are some MNDO calculations on CH3OCNJ
'
Permanent address: Department of Inorganic Chemistry, Technical University, H-1521 Budapest, Hungary. Abstract published in Advance ACS Abstracts, January 15, 1995. @
0022-365419.512099-1649$09.00/0
The higher alkyl- and aryl-substituted cyanates exhibit much greater stability and have been characterized by mass, IR, Raman, and NMR spectroscopy,I6 but the only gas-phase structural study is the microwave investigation of the unstable ethyl cyanate m01ecule.I~ The gas-phase structures of SeF5OCN'* and SF5OCNI9 have been studied recently by electron diffraction, with the only crystal structure determination being that of 4-chloro-3,5-dimethylphenylcyanate.20 Of significant interest for work involving transition metal catalysis is the formation and identification of a surface RhOCN species during the reaction over preoxidized Rl1/Si02.~' In general the available experimental information on cyanates suggests an ether-like valence formulation R-O-CEN, although, as discussed below, the extent of interaction of the R and CN fragments with the central 0 atom must be considered. We have a long-standing interest in the generation, spectroscopy, and structure of small stable and unstable iso~yanates,2~-~~ and so, in the present study, we report the first gas-phase characterization of the unstable methyl cyanate molecule by a combination of ultraviolet photoelectron (PE), photoionization mass (PIMS), and infrared (IR) spectroscopy. Our goals are to establish a procedure for generating pure samples for spectroscopic investigation, to distinguish it from the isocyanate analog, and to provide experimental information on the electronic structure and vibrational spectrum. To this end the electronic and geometric structures and vibrational spectra are supported by ab initio calculations at the correlated level.
Experimental Section A mixture of gaseous methyl cyanate and water was generated by passing 0-methyl thiocarbamate (H3COC(S)NH2) over solid yellow mercury(I1) oxideE packed into a small Pyrex tube. 0-methyl thiocarbamate was prepared according to the literature method;26mercuric oxide was a commercial product (Fisher Scientific). The CH3OCNEI20 mixture from the reaction zone was then passed through a U-tube loosely packed with P2O5. This was cooled with ice/water to assist in trapping excess water. The pure methyl cyanate was then led directly into the photoelectron/PIMS or IR spectrometers. Altemative drying agents lead to the observation of interesting side reactions (see the Results and Discussion) including the formation of the CH3NCO isomer, which, for direct comparison with CHsOCN, was also investigated by PE, IR, and PIMS. The He I photoelectron spectrum was recorded on a fast pumping spectrometer specifically designed to study reactive 0 1995 American Chemical Society
1650 J. Phys. Chem., Vol. 99, No. 6, 1995
Pasinszki and Westwood
TABLE 1: Calculateda Equilibrium Structures of CHJOCNand CHJOCN+ method Cm-0 0-C C-N Cm-HI Cm-Hz CmOC OCN HlCmO H2CmO HICmOC totalenergy 1.453 1.302 1.184 1 .OS2 1.086 113.5 178.4 104.9 109.8 180.0 -207.377 242 CH30CN" MP2 CHJOCN+ X MP2 1.552 1.217 1.208 1.OS2 1.081 123.9 177.3 107.3 102.2 0.0 -206.970 306 1.081 146.2 176.2 A MP2 1.563 1.202 1.230 1.os0 180.0 101.3 103.5 -206.956 383 1.641 1.164 1.216 1.073 159.5 178.2 1.073 99.5 101.3 -206.378 758 CIS 180.0 1.073 1.533 1.191 1.156 1.075 127.2 176.8 101.5 B CIS 180.0 106.6 -206.296 889 c CIS 1.449 1.340 1.174 I .075 1.091 115.2 176.6 180.0 103.9 109.3 -206.240 5 19 1.684 1.169 1.235 1.074 138.9 158.8 1.073 E CIS 180.0 98.1 100.9 -206.203 775 1.525 1.221 1.222 F CIS 1.076 125.1 147.7 0.0 1.076 107.4 103.2 -206.139 73 1 1.409 1.373 1.129 1.248 119.2 179.0 168.6 -206.1 12 51 1 1.095 108.2 115.9 I CIS 1.551 1.191 1.214 1.077 128.8 162.3 1.074 5 CIS -206.077 132 180.0 101.6 104.8 a Bond lengths in angstroms, bond angles in degrees, and total energies in atomic units. Calculated at the MP2(fu11)/6-31G** or CIS(full)/631G** level of theory. "See also ref 12. TABLE 2: Experimental and Calculated Ionization Potentials (eV) of CHJOCN MP2 experimental IP ASCF" Koopmans orb. sym. and character ionic state % 11.30 11.os 11.73 (3a") n,b(OCN) 12.25 11.43 12.22 (12a') n,,b(ocN) A B 13.19 15.12 (lla') n N 14
I 15.5 16.5
15.52 16.09
(2a") Jth(OCN)- CH3 (loa') JCh(0CN)- CH3
c
18.84 19.22
(9a') CH3 + nb(OCN) (la") CH3 + JCb(OCN)
F
D E
e
H
I
5
18.5 2 1.67
(Sa')
CIS ASCF"
assignment
1 I .43'
( 12a')-'
13.76
(1 la')-'
99% 71%
15.41 15.7Sd 16.25
(2a")-' 86% (loa')-' 54% E1 state
18.05 18.08' 18.47f 18.65 19.88
E1 state E1 state (9a')-' 57% ( 1a")-' 42% E1 state
a(0-C)
Vertical ionization potentials. "-Adiabatic ionization potentials; see text for details. e Arbitrary chosen value; see text for details. Did not reach convergence. e Converges to state B. /Decomposes to CH3+ ion and OCN radical. and unstable species. The spectrum was calibrated with the known ionization potentials (IPS) of H20 and N2; resolution was 45 meV during the experiment. This spectrometer, a modification of an earlier version,27not only permits measurement of the electron kinetic energies but can also mass analyze the ions produced in the photoionization process. This is achieved with a quadrupole mass analyzer (Hiden Analytical, 320 amu), mounted directly above the photoionization point. The conventional E1 source of the mass analyzer is removed, ionization being provided by He I (21.22 eV) or unfiltered HL,p,,, (10.2-12.7 eV) radiation. Although not done in coincidence, PE and PIMS spectra can be recorded within seconds of each other; thus it is assumed that for a given PE spectrum the subsequent PTMS is of the same compound. Mid-IR spectra (4000-400 cm-l) were collected on a Nicolet 2OSXC interferometer equipped with a 20 cm single pass gas cell with KBr windows. The effluent from the reaction tube was continuously pumped through the cell using a rotary pump whilst maintaining the pressure between 400 and 500 mTorr. Spectra were collected at 2 or 0.5 cm-l resolution.
Computational Methods Ab initio calculations were performed at the MP2(fu11)/63 1G** or CIS(fu11)/6-31G** level of theory using the Gaussian 92 quantum chemistry package2* implemented on a Silicon Graphics Inc. 4D-380 workstation. Geometry optimizations were performed for the neutral CH30CN molecule and for the ground and 10 lowest excited states of the singly positive ion. Harmonic vibrational frequencies were calculated at the equilibrium geometries to ensure that these geometries were real minima.
Results and Discussion Ab Initio Calculations. The calculated structure of the neutral methyl cyanate molecule and several states of the CH3OCN+ ion are given in Table 1, with the assignment of ionic states collected in Table 2. The calculated (MP2/6-31G**) structure for the ground state neutral is in reasonable agreement with the recently reported r, s t r u ~ t u r e . ~ . ~ ~
Ground State Molecule
Ground State Cation
Particularly important parameters are the C,-0-C angle (calc 113.5", exptl 113.8") and the calculated (and experimental) trans in-plane HIatom for a bent C, structure. The present calculation also supports important parameters obtained in the experimental determination: the calculated 4.5" methyl tilt and the nonlinear cyanate (-OCN) group with a calculated angle of 178.4". This nonlinearity is a common feature for the much investigated isocyanate grouping, where both accurate structure determination and high-level theory generally concur. As noted,g*12 the 0-C bond length (calc 1.302 A) is longer than a typical C=O double bond, but is much shorter than an O(sp")-C(sp) single bond (estimated to be 1.35-1.39 A). This suggests a partial delocalization of the CN n system, e.g. an -O+=C=Nformulation, and so any electronic description must consider a pure ether-like linkage with localized and lone pair orbitals and/or a more delocalized model. The former approach may
Study of the Unstable CH30CN Molecule
J. Phys. Chem., Vol. 99, No. 6,I995 1651
be applied to the analogous thio- and selenocyanates, CH3XCN (X = S, Se), and the latter to CH3NCO. CH30CN is expected somewhere in between. From Table 1 we note that only the X and A ionic states have been calculated at the same level (MP2(fu11)/6-31G**)as the ground state neutral, and appreciable differences in the skeletal structures are seen, commensurate with the removal of n n b electrons; namely, the 0 - C bond length decreases and the C-N bond length increases. The most dramatic changes occur in the ground state ion where the in-plane H atom is now cis to the -CN group (removal of an out-of-plane n n b electron), and also in the A fiist excited ionic state, where the Cm-0-C angle opens up by over 23" (removal of an in-plane Jtnb electron). Although higher ionic states are handled at a different level of theory (CIS), all of the positive ions show bent structures which differ from that of the neutral. Photoionization will thus result in low-intensity adiabatic transitions and broad bands in the PE spectrum. An additional important result of the CIS calculations is that they predict low lying ionic states (8,P, and j), which can originate from the neutral by a simultaneous excitation-ionization process, where one electron of the molecule is excited to one of the empty orbitals and another electron is ejected. These states, often referred to as "shakeup" states are called E1 states in Table 2. We note that geometry optimization for the H ionic state (which has a dominant configuration involving loss of an electron from the CH3 (9a') orbital) leads to rupture of the C,-0 bond and the formation of CH3+ and OCN fragments. The IPS of CH30CN have been calculated at the MP2 level using Koopmans' theorem (vertical IP) and with the ASCF method (adiabatic IP; first two ionic states) for comparison with the experimental IPS(Table 2). The energies of those optimized ions showing stable minima and the optimized neutral molecule have been corrected with the calculated zero-point vibrational energy (zpve) in the ASCF calculations. In the case of the CIS calculations, where the energy of the neutral molecule has not been calculated at the same level of theory, the energy differences between the A and other ionic states were calculated first, and the ASCF IPS were obtained by arbitrarily choosing 11.43 eV (the MP2 ASCF value) as the adiabatic IP of the A state. In the case of ionic states (D, 6,and H) where the equilibrium geometry could not be determined, the energy of the state at the geometry of the neutral molecule was used. Identification. An important aspect of this work is to show that pure methyl cyanate has been generated for spectroscopic investigation and that it is not contaminated with its isocyanate isomer or other decomposition products. To this end the stable CH3NCO isomer was also investigated by PIMS, PE, and IR under the same conditions. As discussed below, the PIMS work is capable of distinguishing the isomers on the basis of the cracking patterns, the IR shows marked differences for the -NCO and -0CN functional groups, and the different PE spectra reflect a fundamental difference in the two molecules; the CH30CN species is quite bent, with a C,-0-C angle of ca. 113", whereas CH3NC0, which has nonrigid behavior, is more linear, with a C,-N-C angle of ca. 136°.30 This difference is then reflected in the relative splitting of the inand out-of-plane p orbitals (mediated by ligand-oxygen interactions) and has important consequences for the electronic spectroscopy. No evidence was found for residual starting material. Photoionization Mass Spectra. The PIMS of CH30CN and its isomer CH3NCO can be seen in Figures 1 and 2, respectively. The molecular peak (M) has a low relative intensity in the He I PIMS of CH30CN and is accompanied by M 1 and M -
I,. . . , . . . . . , !. . , . . . . . . . . . , .. . . . . , . . , . , . . .!!!. , . . , .I 10
b
20
50
60
CH,OCN+
CH,'
e,
+
40
30
co'
OCN'
I
I I
10
20
10
20
30 40 50 60 Figure 1. (a) He1 and (b) HL,,B,~photoionization mass spectra ( m u ) of CH30CN.
a
30
40
50
60
Figure 2. (a) He1 and (b) HL,p,, photoionization mass spectra (amu) of CH3NCO.
1 fragments. It should be noted that these M k 1 fragments are observed with varying relative intensities in both the He I and HLa,p,y PIMS spectra of CH30CN and CH3NCO. These fragments are also seen in conventional electron impact mass spectra of alkyl cyanates (ethyl being the smallest cyanate ~tudied)~' and isocyanates (methyl and e t h ~ 1 ) , 3 ~especially 9~~ the M - 1 fragment, although M 1 is not as strong. The strong, dominant base peak in the CH30CN He I mass spectrum is CH3+. Two other fragments weakly observed at mlz = 28 and 42 correspond to CO+ and OCN+, respectively. As we have shown p r e v i o u ~ l y fragmentation ,~~ is usually minimized with the lower energy HLa,B,y photons, with the molecular peak becoming more favored. Nevertheless, the CH3+ fragment still has a strong relative intensity (Figure lb), indicating that fission of the relatively weak C,-0 bond is the primary fragmentation
+
Pasinszki and Westwood
1652 J. Phys. Chem., Vol. 99, No. 6,1995
5 1 Y
TABLE 3: Experimental and Calculated Vibrational Frequencies (cm-') of CHJOCN
experimental" frequency (3042)d
3600 3200 2800 2400 2000 1600 1200 800
400
Wavenumber (cm-I) Figure 3. Gas-phase infrared spectra of CH3OCN and CH3NCO. Insets show details of CH30CN bands at 2968, 2261, and 1112 cm-I. mechanism. The relative intensity of the CO+ and OCN+ fragments remains low in the HL,,b,? PIMS. The He I PIMS spectrum of the more stable CHsNCO species (Figure 2a) is essentially the same as that obtained by conventional electron impact.32 The base peak is CO+, with CH3NCO+ and CH3+ giving the next two strongest peaks at mlz = 57 and 15, respectively. Two mass peaks unique to this isomer are those at mlz = 29 (CH3N+) and mlz = 27 (believed to be either HCN+ formed by neutral loss of H2CO or C2H3+ formed by neutral NO loss). The HL&p,? PIMS spectrum is, however, somewhat different (Figure 2b). The strong base peak is that of the parent ion, and the only observed fragment is CH3N+ at mlz = 29. Peaks attributable to CH3+ and CO+, which appear in the He I PIMS do not appear at all. The PIMS experiment thus provides a clear distinction between the two isomers, the relative intensities of the major ion peaks, C*H3NO+,CH3+ and CO+ being quite different. The HL,p,? PIMS is especially good for this purpose, since the CH3+ fragment appears only for CH30CN and the CH3N+ fragment appears only for CH3NCO. This difference reflects the relative strengths of the C,-0 and C,-N bonds and the tendency of cyanates to produce hydrocarbon fragments. Infrared Spectrum. The IR spectrum of gaseous CH30CN is in Figure 3. The gas-phase IR spectrum of CH3NCO has been previously reported,34but for direct comparison we show, also in Figure 3, a spectrum of the isocyanate isomer recorded during this work. The vas(NCO)band of CH3NCO centered at 2288 cm-' dominates the spectrum with a calculated intensity of 777.63 km mol-', some 1-2 orders more intense than any other band. Indeed, it does partially overlap v(CN) of CH3OCN centered at 2261 cm-I. However, the cyanate species has, in addition, a distinctive strong band at 1112 cm-I ( v ( 0 CN)) not observed in the isocyanate, and so, as previously noted,6 this latter region is expected to be characteristic for the cyanate group. Corresponding bands, at 2286 and 1081 cm-', have been observed in the IR spectrum of matrix-isolated HOCN.3 The gas-phase IR spectrum of CH30CN concurs in general with that previously obtained in CC4 solution6and shows peaks corresponding to CH stretches (-2970 cm-'), CN stretch (e2260 cm-I), CH3 deformations (-1460 cm-I), CH3 rock (-1220 cm-I), 0-CN stretch (-1110 cm-I), and OC, stretch (-880 cm-'). More specific assignments can now be made by making use of band shapes (Table 3) and the corresponding IR spectra of CH3SCN35and CH3SeCN?6 The calculated unscaled harmonic frequencies for CH30CN (Table 3) are in good agreement with experiment, generally being a few percent too
3017 Q 2979 R 2968 Q, A!B 2959 P 2211 R 2263 Q 2260 Q 2253 P 1470 R 1462 Q, C 1453 P
1221 R 1213 Q, BtA 1205 P 1152 Q, C 1120R 1112Q,A 1105 P 893 R, B 873 P
calculatedb frequency intensity' 3300 (a') 6.36
assignment and description
3263 (a")
13.90
vi CH3 as. str. vI1 CH3 as. str.
3154 (a')
24.62
v2 CH3 sym. str.
2254 (a')
106.48
1561 (a') 1553 (a")
10.28 7.23
1521 (a')
5.61
v5
CH3 sym. def.
1251 (a')
44.90
Yg
CH3 rock
1199 (a")
1.05
1152 (a')
124.71
~3
CN ~ t r . (OCN as. str.)
v4 CH3 as. def.d vi2 CH3
as. def.
vi3 CH3 rock ~7
0-CN
Str.
str.) sym. str.
(C-0-C 918 (a')
24.01
601 (a') 484 (a") 222 (a') 150 (a")
2.39 8.48 7.56 0.01
vg 0-C,
OCN in-plane def. OCN out-of-plane def. vi0 COC in-plane def. vI5CH3rot. vg
vi4
a For the relative intensities see Figure 3. Combination or overtone bands with very weak intensity were observed at 3527, 3356, 3139, 2858, 2310(shoulder), 1756, and 1000 cm-I. Unscaled harmonic frequencies. Calculated at the MP2(fu11)/6-31G** level. In kilometers/ mole. See text.
high, as expected for this level of calculation. We note however that the calculated intensities for v(CN) and v(0-CN) imply that the latter should be slightly stronger, whereas experimentally this is not the case. Such a discrepancy, with a similar level of theory, is also noted for HOCN.3 A significant difference from the HOCN case, which has essentially pure perpendicular or parallel components of the transition moments for the heavy atom modes, is the hybrid nature of many of the bands in CH3OCN, indicative of the C, symmetry and implying that all fundamentals and combination bands are allowed in the IR. With rotational constants8 for this prolate asymmetric rotor ( K = -0.97) of A = 39.042 GHz, B = 5.322 GHz, and C = 4.821 GHz, the theoretical PR separations for pure A-, B-, and C-type bands are 20.8, 17.3, and 31.2 cm-', respectively, calculated using the equations of S e t h - P a ~ l .However, ~~ because of the low symmetry, many of the hybrid bands have somewhat reduced PR separations. The 15 normal modes involve 10 A' and 5 A" species, with the A' modes having A, B, or A B band contours. The three modes of A" symmetry above 600 cm-' have distinct C-type bands, with weak PR branches and strong Q branches. They all occur weakly in the CH3 asymmetric stretch (VI'), asymmetric deformation (VI$, and rock (v13) regions at 3017, 1462, and 1152 cm-', respectively. The other expected C-type bands involving the heavy atom out-of-plane vibration ( ~ 1 4 ) and the CH3 rotation ( ~ 1 5 ) are not observed in this work. The CH3 symmetric stretch ( ~ 2 clearly ) shows an --type band centered at 2968 cm-I with a PR separation of 19.7 cm-' (inset to Figure 3). A weak band at 3042 cm-' is tentatively assigned to V I ,the in-plane CH3 asymmetric stretch, which for thio- and ~ e l e n o c y a n a t e sis~almost ~ , ~ ~ degenerate with the corresponding out-of-plane mode. A similar juxtaposition is obtained for the two CH3 asymmetric deformations, v4 and v12, which are calculated to be essentially degenerate, and yet only the distinct Q branch of the C-type ( ~ 1 2 ) is seen at 1462
J. Phys. Chem., Vol. 99, No. 6, 1995 1653
Study of the Unstable CH30CN Molecule
~
10
12
14
16
18
20
IONIZATION ENERGY (eV)
Figure 4. He1 photoelectron spectrum of CHsOCN.
cm-’. The CH3 symmetric deformation, vg, is not observed, but is predicted to have a low intensity. As mentioned, the strongest band at 2261 cm-I is assigned to the CN stretch, v3, and shows a B-type band exhibiting apparent RQQP structure (inset). The observed PR and QQ separations are 18 and 2.7 cm-’, respectively. It has been suggested7that structure on v(CN) bands of cyanates arises from a Fermi resonance, in this case with 2v7. This mode, best described as a localized C=N stretch, has only a weak contribution from the 0 atom according to the calculations, essentially excluding a more delocalized model with the alternative, but poorer, description of vas(OCN). The band corresponding to the 0-CN stretch (v7)is the next most intense at 1112 cm-’ and has an unusual rotational contour (inset), possibly with incipient RQQP structure. Clearly, both vg and v7 need to be investigated at higher resolution. The frequency calculation indicates that v7 is strongly mixed with the O-Cmefiyl stretch, v8. This latter band with a B-type profile is observed at 893 cm-I. An alternative description for v7 is thus a C-0-C asymmetric stretch. The band at 1214 cm-’ shows B/A character and corresponds to the CH3 rock (%). Overall the band positions and band shapes (see Figure 6 of ref 36) compare favorably with the calculated positions and the corresponding assignments for CH3SCN and CH3SeCN. Several very weak bands are consistently observed (see footnote to Table 3) and are tentatively ascribed to combination or overtone bands. Below 600 cm-’ the bands are predicted to be weak, and with poor signdnoise we can say nothing about this region. Confirmation of this analysis would require deuteration and Raman depolarization measurements, in addition to highresolution work superior to the 0.5 cm-’ resolution used herein. The Photoelectron Spectrum. The PE spectrum of methyl cyanate is shown in Figure 4 and can be compared to the calculated IPS in Table 2. Although the ab initio calculations are qualitatively correct (right sequence of orbital energies versus sequence of ionic states), this has to be regarded as fortuitous as the quantitative agreement is poor, with spacings and absolute energies not correctly reproduced. It is well-known that Koopmans’ theorem does not do well for molecules containing the cyano- g r o ~ p ,or~ indeed ~ , ~ ~for the similar CH3SCN and CH3SeCN molecules.25 We note, however, that the CIS method does approach the correct spacing for the higher ionic states. As mentioned, an interpretation of the electronic structure could consider CH30CN as intermediate between CH3XCN (X = S, Se) with localized CN n and X lone pair orbitals, and CH3NCO where there is a delocalized -NCO fragment perturbed by CH3. We have noted already from the IR results that v(CN) seems very localized, and yet it is apparent from the PE spectrum (and indeed the calculated eigenvectors) that there is considerable delocalization of orbitals. In principle the PE spectrum (Figure 4) should be quantita-
tively different from that of the isomeric CH3NCO species recorded during this work and p r e v i ~ u s l ydue ~ ~to~the ~ ~smaller bond angle resulting in a larger splitting of the in- and out-ofplane n orbitals. However, this is mediated by the interaction between the CN group and the 0 atom which results in some delocalization. The PE spectrum is thus best compared to those of the more stable sulfur and selenium derivatives, CH3XCN.2s,40,41The first two bands in the spectrum of CH30CN at 11.30 and 12.25 eV originate from the two (a’ and a”) nonbonding n orbitals (.7tnb) of the cyanate group. The splitting (0.95 eV) between these bands is much less than that of CH3SCN (2.06 eV) and CH3SeCN (2.20 eV).@ This can be only partly explained by the larger bond angle at oxygen (1 13”) compared to those at S (99°)42and Se (96”)?3 since the splitting is much less than that observed for the in- and out-of-plane oxygen lone pairs (1.5-2.0 eV) in HzO and alkyl ethers.44The increased interaction between the 0 lone pair orbitals and the CN moiety (compared to the S and Se analogs) results in less sensitivity of the now delocalized orbitals to the angle at oxygen. Recent computational studies40confirm that the splitting of the n n b orbitals is less sensitive to the angle at x when x = 0 compared to when X = S or Se. This is attributed to the fact that these orbitals are much more localized on the X atom in the case of CH3SCN and CH3SeCN and is reflected in the first PE bands of CH3SCN and CH3SeCN, which are very sharp and narrow. CH30CN, on the other hand, shows a relatively broad (and hence delocalized) first PE band. The third band in the spectrum of CH30CN at 13.19 eV can be unambiguously assigned to the nitrogen “lone pair” orbital (nN). This band is generally strong and sharp and may be the most characteristic fingerprint of R-X-CN-type molecules. Koopmans’ theorem is especially bad for such a localized orbital, but the assignment of nN to this location is consistent with the most recent assignment of its position in CH3SCN, based on Penning ionization.2s The two broad bands in the 1415.5 and 16.5-18.5 eV regions can be assigned to the bonding orbitals of the OCN group and the methyl group orbitals, respectively. The calculations show that these orbitals, nb(OCN) and CH3, are strongly mixed. The next band is predicted at 21.67 eV (Koopmans); calculations using H M 3 and DFT4s suggest the 8a’ orbital to lie between 18.8 and 19.5 eV, although such a band was not observed in the present work. The CIS calculations indicate that shake-up satellite bands are expected in the 16-20 eV region due to the simultaneous excitation-ionization process (Table 2). The relative intensities of these bands, however, are usually very small in He I PES, and so they are probably hidden under the strong nb(0CN)and n(CHs)-based bands if they appear at all. The relative intensity of these bands generally increases by using photons with higher energy, or metastable atoms for ionization, and so evidence for such structure may best be addressed by means of He I1 PES or Penning ionization spectroscopy. Chemical Reactions of Methyl Cyanate. One of the original aims of this work was to find a method of generating pure CH30CN for spectroscopic examination. Given that the formation of a methyl cyanate/water mixture was achieved recently,*the removal of water seemed the key to the generation of a pure sample. Several drying agents were tried and in most cases (except P205) gave a fast reaction between the metal salts used for this purpose and the gaseous methyl cyanate. When Drierite (Aldrich) with indicator (a mixture of Cas04 (97%) and CoC12 (3%)) held at room temperature was used, the methyl cyanate reacted rapidly with the cobalt salt, forming methyl chloride (eq 1). In addition, the CH3NCO isomer and large amounts of C02 were detected in the gas phase. The
Pasinszki and Westwood
1654 J. Phys. Chem., Vol. 99, No. 6, 1995
formation of C02 could originate from the well-known hydrolysis of methyl isocyanateI6 (eq 2). CH,OCN(g)
+ Cl-(s) - CH,Cl(g) + OCN-(s)
(1)
This secondary reaction could be quenched by cooling the Drierite-containing U-tube with an ice/water mixture. The cobalt chloride was used up in ca. 15-20 min, and after the disappearance of CH3C1, only pure CH3NCO was observed. This reaction was therefore used to record spectra of methyl isocyanate. The use of dry CaC12 instead of Drierite still resulted in the formation of methyl chloride (eq 1) and methyl isocyanate. These experiments indicated that the metal cyanate formed in the initial reaction causes the isomerization of methyl cyanate (eq 3). CH,OCN(g)
+ OCN-(s)-CH,NCO(g) + OCN-(s)
(3)
This kind of reaction has been observed previously between ethyl cyanate and potassium cyanate in aqueous We have also found isomerization using dry MgS04 and dry CaS04. Although we have no evidence, we believe that metal cyanate and monomethyl- or dimethyl sulfate are formed in these reactions, and again the metal cyanate causes isomerization. Earlier work has shown that methyl cyanate is a colorless liquid in the condensed phase at room temperature and is very unstable, isomerizing quickly to methyl i~ocyanate.~Our investigations indicate that the OCN- ion plays a key role in the isomerization, CH3NCO forming instantly in the presence of metal salts; metal salt impurities in a methyl cyanate sample therefore strongly determine its stability. Conclusion For the first time the unstable methyl cyanate molecule has been generated essentially pure by passing 0-methyl thiocarbamate over traps of yellow mercuric oxide and phosphorus pentoxide. Methyl cyanate is very reactive, reacting quickly with metal salts, forming metal cyanates, which can convert the methyl cyanate into the methyl isocyanate. Ab initio calculations at the correlated level show that the ground state CH30CN molecule and the ground and excited state cations have a bent structure with C, symmetry. These calculations have helped to support the photoelectron, infrared, and mass spectra of methyl cyanate and have indicated that two perspectives may be considered with regard to the electronic structure, a discrete ether-like formulation or a more delocalized -0CN model. The spectroscopy of methyl cyanate and the stable methyl isocyanate isomer showed that they are very different from each other, thus making it easy to distinguish between the two isomers. The most characteristic bands of CH30CN for this purpose are those of nN at 13.19 eV in the photoelectron spectrum, the CH3+ peak in the HL,,p,, PIMS, and v,(OCN) at 1112 cm-' in the infrared spectrum.
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