Matrix Infrared Spectra and DFT Computations of 2H-Azirine Produced from Acetonitrile by Laser-Ablation Plume Radiation

Abstract

2H-azirine, a less known acetonitrile isomer, is observed in matrix IR spectra from the precursor exposed to radiation from laser ablation of transition-metals. Its vibrational characteristics confirm the previous results, and those for the deuterated and $^{13}C$ substituted isotopomers are also newly reported. The weak absorptions are traced to the low production yield due to its high energy and low extinction constants. IRC computations reveal smooth inter-conversion between 2H-azirine and $CH_2NCH$, providing a rationale for the observed variation of their relative contents during photolysis.

Introduction

Acetonitrile, which is a common organic solvent and well known for its ability to form adducts with Lewis acids,1 also generates various isomers and fragments.2-6 Many of them have been the subjects of recent spectroscopic and reaction dynamics studies. Especially inter-conversions between the cyano and isocyano isomers, formation of the cyclic derivatives, electron-trapping, and C-H bond dissociation have drawn much attention.2,3,6,7 These less stable species are mostly generated by photolysis of acetonitrile or larger nitrogen containing organic compounds and observed as trapped in cold matrix as well as in gas phase.1-7

Recently it has been demonstrated that these photo-isomers and fragments can also be produced during co-deposition of the precursor with laser-ablation transition-metals.7-9 The radiation from metal plume by laser-ablation is an effective UV source, providing valuable opportunities to prepare the photo-chemical products.7-9 The matrix IR spectra of acetonitrile exposed to the radiation show strong absorptions of CH2CNH and CH3NC,10 which are relatively stable isomers (Figure 1). The production yield of a photo-isomer largely depends on the energy relative to the precursor and other plausible photo-isomers.2,6-8 We have reported observation of CH2NCH absorptions in the matrix IR spectra and suggested the possibility that it is produced via 2H-azirine (cyc-CH2NCH).11

Figure 1.Energies of isomers relative to acetonitrile (computed with B3LYP/6-311++G(3df,3pd)).

Maier et al. have prepared 2H-azirine by photo-dissociation of vinyl azide (CH2CHN3) without information about its isotopomers.2 In this paper, we report observation of 2H-azirine in the matrix IR spectra of CH3CN exposed to the radiation of laser-ablated transition-metals, and possible inter-conversion to energetically comparable CH2NCH. The cyc-CH2NCH absorptions are turned out to be much weaker than those from CH2NCH,11 making them difficult to designate. The vibrational characteristics of the deuterated and 13C substituted isotopomers are also newly observed and correlate well with the DFT results.

 

Experimental

The 2H-azirine spectra shown in this report were recorded from samples prepared by co-deposition of acetonitrile with laser-ablated Hf atoms in excess argon at 10 K using a closedcycle refrigerator (Air Products, Displex).10 However, other metals (groups 3-11 and actinides) also yield the same product absorptions although the intensities vary owing to different laser ablation plume radiation from specific metal surfaces. Therefore, these metal independent absorptions do not arise from metal containing species.

In our experiments, metal atoms and intense radiation from the laser ablation plume impinge on the depositing matrix sample. These methods have been described in detail in previous publications.12 Reagent gas mixtures are typically 0.25-0.50% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto the rotating metal target (Johnson-Matthey) using 5-10 mJ/pulse. After co-deposition, infrared spectra were recorded at 0.5 cm−1 resolution using a Nicolet 550 spectrometer with a Hg-Cd-Te range B detector. Then samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to allow further reagent diffusion.

Complementary density functional theory (DFT) calculations were carried out using the Gaussian 09 package,13 the B3LYP density functional,14 and 6-311++G(3df,3pd) basis sets for H, C, and N to provide a consistent set of vibrational frequencies and energies for the reaction product and its plausible isomers.2,3,11 Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. Additional BPW9115 calculations were done to confirm the B3LYP results. The vibrational frequencies were calculated analytically, and the zero-point energy is included in the relative energy. Intrinsic reaction coordinate (IRC) calculations16 have been performed to link the transition state with the reactant and product.

 

Results and Discussion

2H-Azirine Absorptions. Figures 1-3 show the 2H-azirine (cyc-CH2NCH) absorption regions. The product absorptions are marked “az” (for 2H-azirine), and their frequencies are listed along with the previously reported values and compared with the DFT results in Table 1. The 2H-azirine absorptions are mostly weak as shown in Figures 2-3, reflecting a low production yield. The product absorptions observed in the original deposition spectra remain unchanged on visible (λ > 420 nm) irradiation, slightly increase (5-10%) on uv (240 < λ < 380 nm) irradiation, and gradually decrease in annealing.

Table 1.aFrequencies and intensities are in cm–1 and km/mol. bObserved in an argon matrix. cComputed with B3LYP/6-311++G(3df,3pd). dComputed with BPW91/6-311++G(3df,3pd). 2H-azirine has a Cs structure. eReported by Maier et al. (Ref. 2).

Figure 2.IR spectra in the 2H-azirine (cyc-CH2NCH) absorption regions for CH3CN co-deposited with laser-ablated Hf atoms in excess argon at 10 K and their variation. (a) 0.25% CH3CN in Ar deposited for 1 h. (b) As (a) after photolysis (λ > 420 nm). (c) As (b) after photolysis (240 < λ < 380 nm). (d) As (c) after annealing to 28 K. az stands for 2H-azirine absorption, p indicates precursor absorption, and c denotes common absorption in CH3CN experiments. CH2CNH absorptions are also designated.

Figure 3.IR spectra in the cyc-CD2NCD absorption regions for CD3CN co-deposited with laser-ablated Hf atoms in excess argon at 10 K and their variation. (a) 0.50% CD3CN in Ar deposited for 1 h. (b) As (a) after photolysis (λ > 420 nm). (c) As (b) after photolysis (240 < λ < 380 nm). (d) As (c) after annealing to 28 K. az stands for cyc-CD2NCD absorption, p and M indicate precursor and metal containing product absorptions from Hf + CD3CN reaction. c denotes common absorption in CD3CN experiments. CD2CND and CD3NC absorptions are also designated.

Figure 4.IR spectra in the cyc-13CH2N13CH absorption regions for 13CH3 13CN co-deposited with laser-ablated Hf atoms in excess argon at 10 K and their variation. (a) 0.50% 13CH3 13CN in Ar deposited for 1 h. (b) As (a) after photolysis (λ > 420 nm). (c) As (b) after photolysis (240 < λ <380 nm). (d) As (c) after annealing to 28 K. az stands for cyc-13CH2N13CH absorption, and p and M indicate precursor and metal containing product absorptions from Hf + 13CH3 13CN reaction. c denotes common absorption in 13CH3 13CN experiments. 13CH2 13CNH and 13CH3N13C absorptions are also designated.

Spectral assignments have been done with help from the previously reported frequencies2 and DFT values. The strongest CH2 wagging absorption is observed at 971.2 cm−1 along with its site absorptions at 975.4, 972.6, and 967.2 cm−1, close to the previously reported values (978.7, 977.3, 974.4, and 971.5 cm−1)2 as shown in Table 1. Its D counterpart is newly observed at 781.9 cm−1 (with site absorptions at 784.0 and 780.2 cm−1), and the 13C counterpart at 961.9 cm−1 (with site absorptions at 964.4 and 958.4 cm−1). The C-H stretching absorption at 3078.5 carries its 13C counterpart at 3066.5 cm−1 while the D counterpart is not observed in the congested region. The CH2 anti-symmetric stretching band at 3061.3 cm−1 is accompanied with its 13C counterpart at 3047.2 cm−1. The C=N stretching absorption at 1668.3 cm−1 shifts to 1654.3 and 1635.2 cm−1 on deuteration and 13C substitution.

Similarly the C-C stretching absorptions at 1239.3 cm−1 leads to observation of its D and 13C counterparts at 781.9 and 964.4 cm−1, and the C-H out-of-plane bending absorption at 771.8 cm−1 its D and 13C counterparts at 578.5 and 767.3 cm−1. The CH2 symmetric stretching absorption previously reported at 3005.6 cm−1,2 however, is not discernible in our spectra, being covered by precursor absorption. Newly observed product absorption at 698.5 cm−1 is assigned to the C-N stretching mode with its D counterpart at 631.4 cm−1. The observed frequencies correlate reasonably with the DFT values as shown in Table 1 (0.950-0.980 of the B3LYP frequencies).

These observed product absorptions clearly show that 2H-azirine, a photo-isomer of acetonitrile, is produced along with other isomers and fragments by laser-ablation radiation. They are, however, much weaker than those of previously investigated CH2CNH, CH3NC, and CH2NCH.2,3,11 Computations at B3LYP/6-311++G(3df,3pd) level show that CH2CNH, CH3NC, 2H-azirine, and CH2NCH are 92, 98, 199, and 212 kJ/mol higher in energy than acetonitrile, respectively (Figure 1). The low production yield of this cyclic isomer evidently originates from its high energy.

However, it is still notable that the 2H-azirinee absorptions are weaker than those from energetically comparable CH2NCH.11 DFT results reveal that its absorption constants are considerably smaller than those for CH2NCH as shown in Table 1 (and Table 2 in Ref. 11). For example, the strongest CH2 wagging band of 2H-azirine is eight times weaker than the strongest HCN bending band of CH2NCH. Vibrational analysis indicates that the cyclic structure substantially decreases the amounts of dipole moment change for many of its vibrational modes. For instance, the C=N stretching mode is essentially a CNC anti-symmetric stretching mode for CH2NCH, leading to the second strong band. However, it is mixed with the C-H stretching and CH2 scissoring modes for 2H-azirine, the counteracting dipole moment changes resulting in a weak band.

Molecular Structure and Reactions. Figure 5 shows the B3LYP structures of CH2NCH and 2H-azirine and transition state between them. NBO17 analysis suggests that the two C-N and N-C bonds in CH2NCH are true double bonds (natural bond orders of 1.75 and 1.95), whereas those of 2H-azirine (cyc-CH2NCH) are single and double bonds (natural bond orders of 0.95 and 1.96). The lower bond orders and structural strain in the cyclic configuration lead to the longer bonds in 2H-azirine. The transition state is 183 and 195 kJ/ mol higher than CH2NCH and 2H-azirine, and the single imaginary frequency (591 cm−1) arises from the CNC bending mode, consistent with the fact that the conversion of 2H-azirine to CH2NCH is ring-opening by breaking of the C-C bond.

Figure 5.The B3LYP structures of CH2NCH and 2H-azirine and transition state between them. The bond lengths and angles are in Å and degrees. The numbers in parentheses are the natural atomic charges.17

The energetically comparable CH2NCH and 2H-azirine were previously suggested to be inter-convertible. Intrinsic reaction coordinate (IRC)16 computations are carried out for the isomerization reaction between CH2NCH and 2H-azirine (Figure 6), showing smooth conversion between the two photo-isomers of acetonitrile. These results support the previous assumption that cyclization with H migration from the methyl carbon to the nitrile carbon yields the 2H-azirine from acetonitrile, and following C-C bond dissociation leads to formation of CH2NCH.11 While the CH2NCH absorptions nearly disappear on uv (240 < λ < 380 nm) irradiation after original deposition, the 2H-azirine absorptions increase slightly. This suggests that CH2NCH not only photo-converts to 2H-azirine, but also proceed to other reactions, such as photo-fragmentation to CH2NC in the matrix.5,7

Figure 6.Intrinsic reaction coordinate (IRC)16 calculation between CH2NCH and 2H-azirine.

 

Conclusion

2H-Azirine (cyc-CH2NCH) is produced during deposition of acetonitrile by laser-ablation radiation, and its absorptions are observed in the matrix IR spectra, mostly close to the previously reported frequencies. The absorptions of its deuterated and 13C substituted isotopomers are also newly reported in this work. The observed frequencies and isotopic shifts correlate well with the predicted values. The relatively low absorption intensities are traced to the low production yield due to its high energy and the small absorption constants. IRC computations reveal smooth inter-conversion between 2H-azirine and CH2NCH, the energetically comparable isomers of CH3CN. This corroborates the previous suggestion that 2H-azirine is produced via cyclization and H-migration of acetonitrile, and subsequent dissociation of the C-C bond of the cyclic compound leads to generation of CH2NCH.