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Generation, Spectroscopy, and Structure of Cyanoformyl Chloride and Cyanoformyl Bromide, XC(O)CN Tibor Pasinszki,*,† Gábor Vass,† Dieter Klapstein,‡ and Nicholas P. C. Westwood§ †

Department of Inorganic Chemistry, Institute of Chemistry, Eötvös Loránd University Budapest, H-1117 Budapest, Pázmány P. sétány 1/A, Hungary ‡ Department of Chemistry, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS, Canada B2G 2W5 § Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1 S Supporting Information *

ABSTRACT: Cyanoformyl chloride and cyanoformyl bromide, XC(O)CN (X = Cl and Br), have been investigated in the gas phase by UV photoelectron and midinfrared spectroscopies. The ground-state geometries of the neutral molecules have been obtained from quantum-chemical calculations at the B3LYP and CCSD(T) levels using the aug-cc-pVTZ basis set. The individual spectroscopies provide a detailed investigation into the vibrational and electronic character of the molecules and are supported by quantum-chemical calculations. The results are compared to data for structurally and chemically related molecules.

1. INTRODUCTION Cyanoformyl halides, XC(O)CN, are of considerable interest for both synthetic chemistry and spectroscopy/theoretical chemistry. They have the potential to become carbonylating, cyanating, or cyanoformylating agents in organic chemistry.1−3 However, their chemistry is largely unexplored to date. FC(O)CN was first prepared in 1957 by reacting COF2, NaF, and HCN.1,2 A thermolytic route from 2-chloro-2(chlorothioimino)acetyl fluoride was also developed.3 FC(O)CN is a gas at ordinary conditions with a boiling point of −19 °C and is stable at room temperature in its monomeric form.2,3 FC(O)CN has been suggested to be a powerful fumigating agent for use against roaches, rodents, and other pests.1 ClC(O)CN was first synthesized in 1983;3 it formed upon thermal elimination of SCl2 from 2-chloro-2-(chlorothioimino)acetyl chloride in high yield. Pure ClC(O)CN is stable at −78 °C (boiling point 20 °C), but slow and irreversible disproportionation to phosgene and carbonyl dicyanide takes place at room temperature.3 The formation of ClC(O)CN during the gas-phase thermolysis of dichlorofuroxan has previously been noted by us.4 It is interesting to note that ClC(O)CN has been proposed to be one of the most reactive and toxic intermediates of the metabolism of trichloroacetonitrile in mammals.5 It is of interest because trichloroacetonitrile is often found in chlorinated drinking water.5 The bromo derivative, BrC(O)CN, has never been isolated before. Its formation during the thermal decomposition of dibromofuroxan, however, has been noted by us earlier.6,7 The iodo derivative IC(O)CN is still unknown. Spectroscopic studies of cyanoformyl halides are very scarce to date and have been limited to the IR and photoelectron spectroscopic studies of FC(O)CN.8,9 © 2012 American Chemical Society

In this paper, we report the generation and characterization of cyanoformyl chloride and cyanoformyl bromide, and an investigation of their electronic and geometric structures by quantum-chemical and spectroscopic methods. The latter include ultraviolet photoelectron spectroscopy (UPS) and mid-infrared spectroscopy (IR). Relevant to this work is the previous study of the FC(O)CN molecule8,9 and the corresponding aldehyde, HC(O)CN. The latter has previously been generated by several pyrolytic methods and studied by microwave, infrared, and electronic spectroscopies.10−18

2. EXPERIMENTAL SECTION The specifics of the generation of ClC(O)CN and BrC(O)CN from the precursors dichlorofuroxan, 2-chloro-2(chlorothioimino)acetyl chloride, and dibromofuroxan, respectively, are provided below in the Results and Discussion section. Synthesis of the ClC(O)CN Precursors. Dichlorofuroxan was synthesized by nitration of dichloroglyoxime with fuming nitric acid according to a literature method.19 The purity of the sample was checked by NMR and IR measurements: 13C NMR (CDCl3): 109.8 and 146.4 ppm. For characteristic IR bands see ref 4. 2-chloro-2-(chlorothioimino)acetyl chloride was synthesized according to ref 20 (NMR and IR data are provided in this reference). Synthesis of the BrC(O)CN Precursor. Dibromofuroxan was synthesized by combining and modifying previous procedures21,22 as follows: 8.9 g (0.1 mol) oximinoacetic acid (prepared from glyoxylic acid and hydroxylamine hydrochloride)23 was dissolved in 150 mL of water and cooled to 0−5 °C with ice water. Bromine (0.2 mol, 10.3 mL) was added Received: February 15, 2012 Published: March 12, 2012 3396

dx.doi.org/10.1021/jp301528q | J. Phys. Chem. A 2012, 116, 3396−3403

The Journal of Physical Chemistry A

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lations: CationDoublet keyword used; NState=(10,3). Only valence electrons were correlated in post-HF calculations. All calculations were performed with the Gaussian-09 quantum chemistry package.37 For characterization of the normal vibrational modes of ClC(O)CN and BrC(O)CN, the potential energy distribution (PED), which provides a measure of the internal coordinate contributions, was determined using the BMAT program.38

in portions over a 20 min period and stirred for 30 additional minutes. The cold solution was then extracted with 150 mL of diethyl ether. The organic phase was washed with 100 mL of 0.1 M aqueous sodium sulfite solution. The etheral solution was stirred at room temperature and 5 g of anhydrous magnesium sulfate (in one portion) and 10 g of yellow HgO (in small portions over 30 min) were added. The suspension was stirred for an additional 2 h and then filtered. The solvent was evaporated in vacuo, and the residue obtained was treated with 100 mL of pentane. Evaporating the pentane from the separated solution produced the dibromofuroxan raw product as a yellowish liquid, which solidified upon cooling with ice water. The solid was purified by column chromatography (silica gel, dichloromethane). Yield: 2.1 g (17%) colorless crystalline material. 13C NMR (CDCl3): 98.6 and 137.2 ppm. For characteristic IR bands see ref 24. NMR spectra of the furoxan precursors were recorded at room temperature in CDCl 3 on a Bruker DRX 500 spectrometer using TMS as the internal reference. The IR spectra (resolution 0.5 cm−1) of gaseous ClC(O)CN and BrC(O)CN were collected at room temperature on a Bruker IFS 55 FT-IR spectrometer equipped with a 15 cm single-pass glass cell. The cell, with KBr windows, gave a spectral range from 4000 to 400 cm−1. The effluent from the heated reaction tube or U-trap was pumped continuously through the cell using a rotary vacuum pump while maintaining the pressure between 0.4 and 0.5 mbar. The He I ultraviolet photoelectron spectra (UPS) of the gaseous products were recorded using an Atomki ESA-32 photoelectron spectrometer described in detail elsewhere.25 Photoelectron spectra were recorded using the constant transmission energy mode of the electron energy analyzer and were calibrated with the Ar+(2P3/2,1/2) spin−orbit doublet. The resolution of the analyzer was 30 meV (fwhm for the Ar 2 P3/2 line).

4. RESULTS AND DISCUSSION 4.1. Generation of Cyanoformyl Chloride and Bromide. We have used two routes to ClC(O)CN, the gas-phase thermolysis of dichlorofuroxan 4 and 2-chloro-2(chlorothioimino)acetyl chloride,3 and one route to BrC(O)CN, the gas-phase thermolysis of dibromofuroxan.6 Thermolysis of dichlorofuroxan was carried out at 550 °C in a quartz tube (8 mm i.d.) heated along 20 cm; for a more efficient pyrolysis, the tube was loosely packed with quartz glass chips. The pyrolysis products (Scheme 1) were trapped with Scheme 1

3. COMPUTATIONAL METHODS The HF wave functions of ClC(O)CN and BrC(O)CN were found to be unstable (RHF-UHF instability) and so MPn calculations were rejected in this work. Single-point CCSD(T) and UCCSD(T) calculations provided identical total energies. The B3LYP wave functions were stable. CASSCF(10,10)// B3LYP single-point calculations indicated that the weight of the main HF configuration was 93% and 91% for the chloro and bromo derivatives, respectively, and the weight of any other configuration was smaller than 2%. The CCSD T1 diagnostics26 for ClC(O)CN and BrC(O)CN, calculated at the CCSD(T) geometries, were 0.016 and 0.017, respectively. CASSCF calculations and T1 diagnostics thus suggest that single-reference post-HF methods are sufficient for describing the structure of both molecules. The geometries of the ground-state neutral molecules were calculated at the B3LYP27,28 and CCSD(T)29 levels, and harmonic vibrational frequencies were obtained to identify them as real minima (zero imaginary frequencies) on the potential energy surfaces. Anharmonic vibrational frequencies were calculated at the B3LYP level within the framework of second-order vibrational perturbational theory.30,31 Calculations were done using the correlation consistent aug-cc-pVTZ basis sets on all atoms.32 Vertical ionization energies (IEs) were calculated using the outer valence Green’s function (OVGF)33,34 and the symmetry-adapted cluster/configuration interaction (SAC-CI) method.35,36 Details of SAC-CI calcu-

liquid nitrogen in a U-trap. The trap was then isolated and gradually warmed, and the pyrolysis products were detected by spectroscopy according to their volatility. The most volatile products, CO, NO, Cl2, and CO2, were pumped off at a temperature below −100 °C. ClC(O)CN was slightly less volatile than Cl2CO and more volatile than ClCN. Cl2CO was thus completely pumped off at a U-trap temperature of ca. −80 °C before recording the IR and UPS spectra of ClC(O)CN. A ClCN-free ClC(O)CN sample, however, could not be obtained. The IR and UPS spectra of ClC(O)CN were thus obtained by digitally subtracting the spectrum of pure ClCN, recorded separately, from that of the ClC(O)CN and ClCN mixture. After fully evaporating ClC(O)CN and ClCN from the trap and increasing the temperature, a small amount of tetrachloroethylene was also detected, which was the least volatile component of the pyrolysis product mixture. ClC(O)CN was also prepared by the flash-vacuum pyrolysis of 2-chloro-2-(chlorothioimino)acetyl chloride3 by passing the room temperature vapor through a 16 mm i.d. quartz tube heated to 550 °C over a length of 33 cm. The pyrolysis products were pumped through a liquid-nitrogen-cooled trap. The total condensates were subjected to low-temperature vacuum fractionation monitored by gas-phase infrared spectroscopy. Besides the expected reaction byproduct sulfur dichloride, the IR analysis also identified OCS, HCN, COCl2, 3397



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Table 1. Calculated Equilibrium Structures, Energies, Dipole Moments, and Rotational Constants of ClC(O)CN and BrC(O)CNa ClC(O)CN X−C CO C−C CN XCO OCC CCN total energy μ Ae Be Ce

BrC(O)CN b

CCSD(T)

B3LYP

1.753 1.192 1.464 1.163 124.1 124.1 179.3 −665.583059 1.77 7.2165 3.1815 2.2080

1.769 (1.689) 1.184 (1.231) 1.453 (1.413) 1.150 (1.156) 123.8 (112.6) 124.6 (123.3) 179.0 (178.5) −666.461128 1.52 7.2338 3.1997 2.2184

CCSD(T)

B3LYPb

1.921 1.193 1.463 1.164 123.9 124.2 179.6 −2778.493660 1.92 6.2940 2.1300 1.5914

1.940 (1.958) 1.184 (1.191) 1.451 (1.408) 1.151 (1.155) 123.7 (106.2) 124.9 (132.8) 179.6 (177.4) −2780.455030 1.64 6.3355 2.1296 1.5939

a

Bond angles in degrees, bond lengths in angstroms, total energies in atomic units, dipole moments in debyes, and equilibrium rotational constants in gigahertz. Isotopes: 79Br, 35Cl, 12C, 16O, 14N. See Figure 1 and Table S1 (Supporting Information) for further details. bThe geometry of the ground-state cation is provided in parentheses. The cation has Cs symmetry.

dipole moments and rotational constants. The alignments of the two in-plane inertial axes are shown in Figure 1.

and ClCN as impurities (see Scheme 1). Samples of cyanoformyl chloride, with a reported boiling point of 20 °C,3 could be stored for several days at −15 °C in evacuated glass containers fitted with Teflon valves. BrC(O)CN was generated by flash-vacuum pyrolysis of dibromofuroxan in the gas phase at 500 °C (quartz tube (8 mm i.d.); heated along 20 cm; loosely packed with quartz glass chips). Before condensation of the pyrolysis products (see Scheme 1), HBr was added to the effluent of the reaction tube in order to remove ONCCNO from the gas phase in the form of nonvolatile dibromoglyoxime. ONCCNO is known to be explosive in the solid phase at temperatures above −45 °C.6 The gaseous products were then trapped with liquid nitrogen. The U-trap was then isolated and gradually warmed. The most volatile products, CO, NO, and CO2, were pumped off at a temperature below −100 °C. The next most volatile component was BrC(O)CN, which emerged from the trap together with Br2 (detected by UPS). Since a Br2-free sample could not be obtained, the UPS spectrum of BrC(O)CN was obtained by digitally subtracting the UPS spectrum of Br2, recorded separately, from the spectrum of the BrC(O)CN and Br2 mixture. The U-trap was held at −75 °C during the UPS spectroscopic measurement. In order to provide sufficient vapor pressure for IR, the U-trap temperature was increased to −60 °C during the IR spectroscopic measurement, and weak bands due to small amounts of less volatile side products, BrCN, BrCCBr, and Br2CO, were also detected. The effluent from the −60 °C U-trap was continuously monitored and the IR spectrum was recorded after IR bands of BrCN and BrC CBr completely disappeared. The IR spectrum indicated that this final BrC(O)CN sample still contained a small amount of Br2CO impurity. After BrC(O)CN and Br2CO were fully evaporated from the trap and the temperature was increased, a small amount of Br2CCBr2 was also detected, which was the least volatile component of the pyrolysis product mixture. 4.2. Equilibrium Structures of Cyanoformyl Chloride and Bromide. The structural data for cyanoformyl chloride and bromide are presented in Table 1, calculated by the B3LYP and CCSD(T) methods. The ground-state structures of both molecules are planar, with Cs symmetry, and are almost identical, except for the C−halogen bond. Table 1 and Table S1 (in the Supporting Information) also contain the predicted

Figure 1. Structures of cyanoformyl chloride and cyanoformyl bromide, and their orientation in the principal axis system.

Noteworthy is the orientation of the CO bond relative to the a- and b-axes for the two species: it is almost parallel with the b-axis for the chloride but between the a- and b-axes for the bromide. Thus, the (highly localized) CO vibrational motion should produce an almost pure B-type band contour in the IR spectrum of the chloride, but an AB-hybrid band contour for the bromide. The rotational constants, which predict the molecules to behave as near prolate asymmetric tops, can be used as starting points in the analysis of higher resolution molecular spectra of these species. Asymmetric top parameters are provided in Tables 2 and 3. 4.3. Infrared Spectra of Cyanoformyl Chloride and Bromide. The nonlinear planar structure of the cyanoformyl halides, belonging to the Cs molecular point group, results in the species having nine normal modes of vibration, seven of which are in the molecular plane (a′) and two which are out-ofplane (a″). All vibrational modes are both infrared and Raman active. The gas-phase IR spectra of cyanoformyl chloride and cyanoformyl bromide are shown in Figure 2 and on expanded scales in Figures S1 and S2 in the Supporting Information. The vibrational frequencies of the normal modes were calculated by ab initio methods at the CCSD(T) level and by DFT methods at the B3LYP level. The calculated values, and the potential energy distributions of the modes, are given in Tables 2 and 3, along with the assigned experimental values. The calculated values indicate that at least six, and perhaps seven, of the fundamentals should give rise to infrared bands 3398



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Table 2. Experimental and Calculateda Vibrational Frequencies (cm−1) of ClC(O)CN B3LYP exptl freqb 3525 (vw) 2242 Q (m) 1774 (vs) 1074 Q (w) 1025 Q (m) 986 Q (vs) 907 (vw) 677 Q (vw) 654 Q (m) 510 Q (w) 422 (vw) − −

CCSD(T) freqc 2234 (a′) 1764 (a′)

959 (a′) 648 647 488 416 245 162

(a″) (a′) (a′) (a′) (a″) (a′)

freqd

intensitye

3564 (3666) 2311 (2342) 1795 (1833) 1075 (1085) 974 (1007) 938 (987) 897 (925) 668 (675) 655 (663) 481 (503) 417 (422) 262 (266) 170 (173)

PEDf

assignment and description 2ν2 v1 CN stretch v2 CO stretch v4 + v6 2ν5 v3 C−C stretch v5 + v6 v8 Cl oop wag v4 CCO bend v5 Cl−C stretch v6 ClCO bend v9 C−C(N) tors v7 CCN bend

33 293

304 5 29 35 3 16 5

CN(91) CO(92)

CC(37), ClC(15), ClCO(22), CCO(21) Cl wag(84), C−C(N) tors(16) CCO(43), ClC(21), CC(18), CCN(13) ClC(70), CCN(19) ClCO(72) C−C(N) tors(79), Cl wag(21) CCN(57), CCO(40)

Unscaled anharmonic frequencies. Isotopes: 35Cl, 12C, 16O, 14N. Asymmetric top parameters (based on B3LYP geometry): κ = −0.609, σ = 9.222. Gas phase. Position of the most intense Q-band or the band center is given. See the Supporting Information for band-shape details. cCCSD(T) harmonic vibrational frequencies corrected with B3LYP anharmonic contributions. dB3LYP anharmonic frequencies. Harmonic frequencies are in parentheses. eIn km/mol. Calculated using the harmonic force field. fPotential energy distribution from force field analysis based on B3LYP force constants. a b

Table 3. Experimental and Calculateda Vibrational Frequencies (cm−1) of BrC(O)CN B3LYP exptl freqb 3514 (vw) 2236 Q (m) 1920 (vw) 1769 Q (vs) 1007 Q (w) 959 Q (vs) 634 (w) − 412 (w) − − −

CCSD(T) freqc 2226 (a′) 1721 (a′) 939 629 619 415 368 247 144

(a′) (a′) (a″) (a′) (a′) (a″) (a′)

freqd 3546 (3664) 2304 (2336) 1935 (1984) 1760 (1832) 1008 (1014) 920 (959) 644 (650) 639 (646) 412 (418) 361 (364) 261 (262) 150 (152)

intensitye

PEDe

assignment and description 2v2 v1 CN stretch ν2 + ν7 v2 CO stretch ν4 + ν6 v3 C−C stretch v4 CCO bend v8 Br oop wag v5 Br−C stretch v6 BrCO bend v9 C−C(N) tors v7 CCN bend

31 324 286 23 3 40 2 15 4

CN(91) CO(93) CC(42), BrC(11), BrCO(20), CCO(22) CCO(51), CCN(17), CC(15), BrC(11) Br wag(83), C−C(N) tors(17) BrC(41), BrCO(24), CCN(28) BrCO(52), BrC(35) C−C(N) tors(79), Br wag(21) CCN(53), CCO(40)

Unscaled anharmonic frequencies. Isotopes: 79Br, 12C, 16O, 14N. Asymmetric top parameters (based on B3LYP geometry): κ = −0.774, σ = 16.700. Gas phase; additional very weak bands at 869 and 825 cm−1. Position of the most intense Q-band or the band center is given. See the Supporting Information for band-shape details. cCCSD(T) harmonic frequencies corrected with B3LYP anharmonic contributions. dB3LYP anharmonic frequencies. Harmonic frequencies are in parentheses. eIn km/mol. Calculated using the harmonic force field. fPotential energy distribution from force field analysis based on B3LYP force constants. a b

above the 400 cm−1 cutoff of the instrument used in these experiments. The highest energy fundamental for both species is ν1(C N) which is centered (Q branch) at 2242 cm−1 for the chloride and 2236 cm−1 for the bromide. The bands exhibit hot band Qstructure to lower energies. The presence of the electronwithdrawing carbonyl and halide groups weakens the intensity of the band,39 so that while it is characteristic, it is not a dominant band in the IR. The carbonyl stretching mode ν2 gives rise to strong IR bands. For the chloride a strong B-type band is observed centered at 1774 cm−1. This compares well with the ν(CO) value of 1767 cm−1 found for propynoyl chloride40,41 and intermediate to the values for cyanoformyl fluoride (1857 cm−1)8 and cyanoformaldehyde (1716 cm−1).17 The ν2(CO) band of the bromide is slightly lower in energy at 1769 cm−1, in keeping with the decreased electronegativity of the halogen substituent. The bromide ν2 band also displays a characteristic

A/B hybrid contour, consistent with the orientation of the C O bond relative to the a and b inertial axes predicted by the structural calculations. The most intense bands in the IR spectra are at 986 cm−1 for the chloride and 959 cm−1 for the bromide and assigned as ν3. These arise from a vibration that is nominally labeled as ν(C− C) but which the potential energy distributions of the normalmode analyses show to have significant contributions from other atomic motions. A better description for this mode would be an asymmetric combination of the C−C and C−X stretching motions. While the coupling between the two motions is probably not as large as proposed for cyanoformyl fluoride,8 where the C−C and C−F vibrations are very similar in energy, leading to large coupling, it is not negligible for the chloride and bromide. This is revealed by the potential energy distributions (Tables 2 and 3). The shift of the band from 986 to 959 cm−1 for the bromide reflects the decreasing contribution of the C−X 3399



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molecular orbitals (MOs) from HF calculations for the Cl and Br derivatives, respectively, are .....(5a′)2 (6a′)2 (1a″)2 (7a′)2 (8a′)2 (9a′)2 (2a″)2 (3a″)2 (10a′)2 .....(5a′)2 (6a′)2 (1a″)2 (7a′)2 (8a′)2 (2a″)2 (9a′)2 (3a″)2 (10a′)2

Molecular orbital plots for both derivatives are shown in the Supporting Information. The He I photoelectron spectra of the cyanoformyl halides are shown in Figure 3, and experimental and calculated

Figure 2. IR spectra of cyanoformyl chloride and cyanoformyl bromide. Br2CO impurity bands are marked with asterisks.

motion to the mode as the C−X frequency becomes further removed from that of the C−C motion. It is noteworthy that the ν3 bands of both the chloride and the bromide show a satellite band 40−50 cm−1 to higher energy. A hot band explanation is disfavored because the satellites appear to higher energy, and the thermal population of an excited vibrational level of this relatively high-energy mode is expected to be small. The satellites are assigned to overtone and combination bands which gain intensity via Fermi resonance with the nearby, intense, ν3 bands. For the chloride the 1074 cm−1 satellite band (B3LYP calculated value 1075 cm−1) and for the bromide the 1007 cm−1 satellite band (calculated value 1008 cm−1) is assigned as a ν4 + ν6 combination band. The assignment of the relatively intense 1025 cm−1 satellite band of the chloride may be assigned to the overtone of ν5 (Cl−C stretch). The bands in the low-frequency region of the spectra are all of low intensity. The calculations predict ν4, the in-plane C− CO bending, and ν8, the out-of-plane C−X wagging, to be very close in energy for both species. For the chloride, ν8 is assigned to a weak shoulder at 677 cm−1 and ν4 to a stronger band at 654 cm−1 on the basis of the calculated frequencies and intensities. The corresponding bands of propynoyl chloride40,41 are found at 665 and 655 cm−1, respectively. For cyanoformyl bromide the band due to ν4 has been identified at 634 cm−1 with ν8 not located (a very small calculated intensity). The C− Cl stretch (ν5) of the chloride is assigned to the band at 510 cm−1 and the in-plane OC−Cl bending mode (ν6) produces a very weak feature at 422 cm−1. The C−Br stretch for the bromide (ν5) is observed at 412 cm−1, whereas the OCBr bend (ν6) is predicted to be very weak and below the KBr window cutoff. 4.4. Photoelectron Spectra of Cyanoformyl Chloride and Bromide. The ground-state electronic structures of both cyanoformyl halides investigated are 1A′. The sequences of

Figure 3. He I photoelectron spectra of cyanoformyl chloride and cyanoformyl bromide.

ionization energies are listed in Table 4. SAC-CI and OVGF calculations for vertical IEs give good agreement with experiment. We note, however, that SAC-CI calculations are superior to OVGF, especially in the low IE region. From the calculated IEs and from a comparison of the He I spectra with those of cyanoformyl fluoride,9 cyanoformyl aldehyde,16 and acetyl cyanide42 the band assignments are relatively straightforward. The low ionization energy (IE) regions for both cyanoformyl halides are expected to be dominated by bands associated with the in-plane orbital of the carbonyl group (n′O) and the in- and out-of-plane halogen lone-pair orbitals (n′Hlg and n″Hlg). The chloride exhibits a rather broad lowest IE band followed by a very intense band roughly twice the intensity of the first. As the chlorine lone-pair orbitals are expected to be very close in energy, the second, intense, band at 13.07 eV can be assigned to ionization from the n′Cl and n″Cl orbitials, with the lowest IE band at 12.30 eV assigned to photoionization from the n′O orbital. The SAC-CI calculations predict that the n′O band has a lower energy than those due to the n′Cl and n″Cl ionizations, the latter two having very similar energies. The experimental assignments are supported by comparison with values for other acyl chlorides. For acetyl chloride,43 propynoyl chloride,44 and 3400



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Table 4. Experimental and Calculated Vertical Ionization Energies (eV) of ClC(O)CN and BrC(O)CN ClC(O)CN expt 12.3 13.07 13.74 14.02e 14.43 16.39f 17.12g 18.1

OVGFa 12.90 12.81 13.18 13.67 13.90 14.77 16.76 17.70 18.72

(9a′) (10a′) (3a″) (2a″) (8a′) (7a′) (1a″) (6a′) (5a′)

BrC(O)CN SAC-CIa,b,c

12.11 12.92 13.08 13.75 14.01 14.54 16.70 17.43 18.55

[0.77(9a′), 0.56(10a′)] [−0.76(10a′) 0.51 (9a′)] [−0.94(3a″), 0.25(2a″)] [0.94(2a″), 0.24(3a″)] [0.91(8a′), 0.26(9a′)] [0.94(7a′)] [0.94(1a″)] [0.90(6a′), 0.30(5a′)] [0.89(5a′), −0.29(6a′)]

expt

OVGFa

SAC-CIa,b,d

orbital character

11.8 12.07 12.36 13.45 13.69h 14.26i 15.9 16.8 17.7

12.40(9a′) 11.94 (10a′) 12.11 (3a″) 13.64 (2a″) 13.60 (8a′) 14.66 (7a′) 16.43 (1a″) 17.14 (6a′) 18.37 (5a′)

12.07 [0.70(9a′), 0.64(10a′)] 11.67[−0.73(10a′), 0.60(9a′)] 12.11 [0.97(3a″)] 13.69 [−0.97(2a″)] 13.77 [−0.93(8a′), −0.25(9a′)] 14.39 [0.94(7a′)] 16.41 [0.94(1a″), 0.11(2a″)] 17.02 [0.91(6a′), −0.22(5a′)] 18.14 [−0.92(5a′), −0.20(6a′)]

n′O n′Hlg n″Hlg π″(CN) π′(CN) n′N π″(CO) σ′ σ′

CCSD(T)/aug-cc-pVTZ geometries of neutral molecules were used. bOpen-shell occupancy for single excitations and SAC-CI coefficients (|ci| > 0.2) are provided in parentheses. c“Shake-up” bands are predicted to be below the detection limit at 21.62, 22.32, and 22.53 eV. d“Shake-up” bands are predicted to be below the detection limit at 21.25, 21.36, 21.49, and 21.66 eV. eVibrational structure: 2040, 430 cm−1 (±50 cm−1). fAdiabatic IE: 16.18 eV. Vibrational structure: 1710, 570 cm−1 (±50 cm−1). gVibrational structure: 1360 cm−1 (±50 cm−1). hVibrational structure: 1890, 410 cm−1 (±50 cm−1). iVibrational structure: 850 cm−1 (±50 cm−1). a

formyl chloride,45 the n′O band is observed at 11.08, 11.25, and 11.61 eV, respectively, while the two nCl bands are both observed at 12.00, 12.31/12.68, and 12.38/12.46 eV, respectively. The variation in ionization energies is the direct result of the inductive effects of the other substituents on the acyl chloride group, the ionization energies following the trend −CH3 < −CCH < −H < −CN. The broadness of the n′O band is common to all the above-cited acyl chlorides and indicates that there is a substantial geometry change between the cationic state and neutral ground state. As shown in Table 1, there are significant changes in the XCO bond angles in both ground-state cations, especially noticeable for BrC(O)CN where there is a pronounced change in both the BrCO and OCC angles even though planarity is retained. The MO calculations reveal appreciable contributions of halogen atomic orbitals to the n′O MO (see Supporting Information, Figures S3 and S4); thus, it is not surprising that removal of an electron from this MO results in a geometry change, from both its bonding characteristics and its angle-defining properties. For cyanoformyl bromide, the 11−13 eV ionization energy region should also show three ionizations: n′O, n″Br, and n′Br. Compared to the chloride, the bromine lone-pair bands should be at lower ionization energies. The observed spectrum for the bromide thus displays strongly overlapped bands in this region, at 11.80, 12.07, and 12.36 eV. Based on the band profile, the first band at 11.80 eV is assigned to the n′O ionization. This value is 0.5 eV less than that of the chloride, reflecting the decreased inductive effect of the less electronegative bromine substituent. A similar destabilization of 0.4 eV was observed for acetyl bromide relative to acetyl chloride.43 The n′O band of cyanoformaldehyde16 is observed at 11.91 eV. The intense bands at 12.07 and 12.36 eV for the bromide are thus assigned to ionizations from the n′Br and n″Br orbitals, respectively. The splitting of the bands, 0.29 eV, may be attributable to several reasons. The n′Br orbital may be stabilized by interaction with the lower IE n′O orbital, the n″Br orbital may be destabilized by interaction with the higher IE π″CN orbital, or it may be due to spin−orbit splitting. Even though the Cs molecular symmetry removes the degeneracy of the nBr orbitals, Heilbronner has shown that nBr orbital splittings in such cases are similar to those due to spin−orbit coupling.46 This explanation is supported by the observation that the splitting here, 0.29 eV, is very similar to the spin−orbit splitting of 0.32 eV found for hydrogen bromide.47 Note that the broadness of the combined

n′Cl, n″Cl band of the chloride obscures any such splitting, expected to be similar to the value of 0.10 eV observed for hydrogen chloride.47 The 13−15 eV IE region of the spectra should contain bands due to ionizations from the π″CN, π′CN, and n′N orbitals. For hydrogen cyanide the degenerate πCN bands appear at 13.60 eV while the n′N band is at 14.0 eV;47 for acetonitrile (methyl cyanide) the corresponding bands are at 12.20 and 13.14 eV.48 Addition of an electronegative carbonyl group, and then a halogen substituent, should result in substantial shifts to higher ionization energies. For cyanoformaldehyde, the π″CN band is observed at 13.51 eV while the π′CN band has been assigned a value of 14.27 eV;16 the degeneracy of the πCN orbitals is removed by interaction of π″CN with π″CO (at higher IE). However, based on the experimental results for cyanoformyl fluoride and MO calculations, it has been suggested9 that for cyanoformaldehyde both the π″CN and π′CN ionizations appear at 13.51 eV and the n′N ionization produces the 14.27 eV band. For cyanoformyl fluoride9 the π″CN band is observed at 13.90 eV, π′CN at 14.14 eV, and n′N at 14.76 eV. The spectra of both the chloride and bromide species similarly show three distinct bands in the 13−15 eV region. Based on MO calculations these three bands for both species are assigned to π″CN, π′CN, and n′N orbitals in order of increasing IE (see Table 4). The shift of the corresponding bands can be well explained by the increasing electronegativity of the substituent atom, viz., Br < Cl < F. Thus far, two of the three out-of-plane orbital bands (n″X, π″CN, π″CO) for the two species have been identified. The π″CO should lie at higher ionization energy. For cyanoformaldehyde the π″CO band has been assigned to a band at 17.17 eV.15 However, MO calculations and comparison with the spectrum of cyanoformyl fluoride9 suggested that an experimental IE observed at 15.88 eV would be a more reasonable value for the π″CO band in the aldehyde. For cyanoformyl chloride and bromide, the π″CO ionization should be at higher IE compared to the aldehyde (inductive effect). For the chloride, π″CO is assigned to the 16.39 eV IE band and for the bromide to the band at 15.95 eV. The aldehyde (revised) to chloride shift is consistent with that observed in the acetyleneformyl series:44 propynal has π″CO at 14.37 eV while for propynoyl chloride it is at 15.31 eV. The generally higher IEs found here for the cyanoformyl series again reflects the increased inductive effect of the cyano group relative to the 3401



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splittings. Of the cyanoformyl species studied thus far (X = H, CH3, F, Cl, Br), only in the F-species does the halogen exert an observable conjugative effect.

acetylenic group. The assignments are consistent with the calculated values, found by both SAC-CI and OVGF methods. The contours of the π″CO band in all the aforementioned examples are all broad, sometimes allowing resolution of extensive cationic vibrational structure. This is partially resolved for the chloro derivative with an adiabatic IE at 16.18 eV with vibrational structure of 1710 and 570 cm−1 (Table 4). As noted earlier and in Table 1, there must be considerable geometry differences between the ground state of the neutral and the corresponding cationic excited state; an aplanar excited state is a possibility. The two highest IE bands observed in the spectra of the chloride and bromide are assigned to ionizations from in-plane σ-bonding orbitals. The calculations predict the 17.12 eV band of the chloride to arise from ionization from an orbital with significant density in the O−C−Cl internuclear regions. A band with a similar, sharp, profile is observed at 16.86 eV for propynoyl chloride.44 The PE assignments can be summarized in a correlation diagram, Figure 4, which compares the assignments with those

5. CONCLUSION Cyanoformyl chloride and bromide have been generated by gas-phase thermolysis reactions. The IR and valence-shell photoelectron spectra have been obtained for the gas-phase species. The vibrational data have been interpreted with the aid of computational results and comparison with related species. The photoelectron data, coupled with quantum chemical calculations, indicates some delocalization of the carbonyl and cyano π-systems. The influence of the chloro- and bromosubstituents on the electronic structures was found to be mainly of an inductive nature.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, S2, and S3 list the calculated rotational constants, vibrational frequencies, and IR intensities of ClC(O)CN and BrC(O)CN isotopomers. Figures S1 and S2 show selected bands of the IR spectrum of ClC(O)CN and BrC(O)CN on an expanded scale. Figures S3 and S4 show molecular orbital pictures of ClC(O)CN and BrC(O)CN. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.P. and G.V. thank the European Union and the European Social Fund for financial support to the project under the grant agreement no. TÁ MOP 4.2.1./B-09/1/KMR-2010-0003 and the Hungarian Scientific Research Fund (grant no. OTKA K101164). D.K. and N.P.C.W. thank the Natural Sciences and Engineering Research Council of Canada for Discovery and Equipment grants in support of this work. D.K. also thanks the St. Francis Xavier University Council for Research for support.

Figure 4. Experimental ionization energy correlation diagram for cyanoformyl halides and related molecules.

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of structurally related compounds. Within the approximations of Koopmans’ theorem, the diagram then shows the interaction and inductive effects on the orbital energies. Starting from formaldehyde, it can be seen how the substitution of the hydrogens, first with a halogen and then additionally with a cyano group, causes significant stabilizing shifts of the common orbital energies, resulting in observed larger ionization energies. Conversely, starting from hydrogen cyanide, addition of an acyl halide group introduces substituents capable of exerting both inductive and conjugation effects. The energy difference of the in- and out-of-plane πCN bands is a measure of the interaction of the π″CN and π″CO orbitals. That the splitting observed for cyanoformyl chloride (0.28 eV) is larger than that observed44 for the πCC splitting of propynoyl chloride (0.05 eV) indicates that in these isoelectronic species π″CO interacts to a larger extent with π″CN than with π″CC. This may be the result of π″CO and π″CN being energetically more similar, causing larger perturbations. On the other hand, the effects of the large halogen atoms appear to be predominantly inductive as the halogen lone-pair orbital bands do not show any increased

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