Conformational isomerism in methyl cyanoacetate: A combined matrix-isolation infrared spectroscopy and molecular orbital study

Abstract

Study of the conformational isomerism of methyl cyanoacetate MCA) aided for the Ðrst (N3CCH 2 COOCH 3 , time by matrix-isolation infrared spectroscopy is reported. The conformational isomerization processes in MCA in the temperature range from 10 to 70 K were studied in detail in argon and xenon matrixes. During annealing of the matrixes direct interconversion of the gauche into the syn conformer has been registered. A similar, but more pronounced, gauche ] syn interconversion e †ect was observed in a series of experiments in which matrixes were deposited at increased substrate temperature. The experiment is supported by theoretical predictions undertaken at di †erent levels of approximation (MP2 and DFT/B3LYP). It is shown that, for the single molecule in vacuum, the syn conformer (CÈCÈC2O dihedral angle equal to 0¡) corresponds to the conformational ground state, with the doubly degenerated by symmetry gauche conformer (CÈCÈC2O dihedral angle equal to^139.9¡) being only slightly less stable than the most stable form (MP2/6-31G** kJ mol~1). In the matrixes, the energy gap between the syn and gauche conformers *E (gauche?syn) \ 0.174 increases, with estimated to be about 1.4 kJ mol~1 in xenon. The predicted energy barriers for *E (gauche?syn) conformer interconversion were found to be signiÐcantly low : the MP2/6-31G** calculated *E (syn?gauche) energy barrier is ca. 3.65 kJ mol~1, while the calculated energy barrier separating the two symmetrically equivalent gauche conformers stays only 0.196 kJ mol~1 above the energy of these forms. In the matrixes, energy barrier increases to at least 17È18 kJ mol~1, while the energy barrier is, *E (syn?gauche) *E (gauche?gauche) with all probability, still lower than in the gaseous phase. The matrix isolation spectroscopic data indicate that interconversion between the gauche conformers occurs even in the low temperature matrixes.

Matrix-isolation and solid state low temperature FT-IR study of 2,3-butanedione (diacetyl)

“…The local order is also higher in the case of the matrix experiment, as they are the packing requirements, since the arriving molecule faces an essentially crystalline media (argon crystal), while in the case of deposition of the amorphous layer it faces an essentially disordered molecular environment. It is well known that in general more planar structures are stabilized in matrices [43,44]. On the other hand, the local electric field is considerably more important for the amorphous layer.…”

Low Temperature Infrared Spectroscopy Study of Pyrazinamide: From the Isolated Monomer to the Stable Low Temperature Crystalline Phase

“…Indeed, with such a low energy barrier for Z f E conversion, the residual amount of Z molecules existing in the gas phase prior to deposition can be expected to rapidly convert to the most stable form during deposition of the matrix. [43][44][45][46][47] Because of the excellent agreement between the theoretical and experimental spectra (Figure 5), the general assignment of the experimental bands was straightforward (Table 2). Interestingly, the detailed analysis of the experimental spectra revealed that PZA molecules occupy three main matrix sites in the argon matrix, with most of the bands appearing split into triplets.…”

Missing conformers. Comparative study of conformational cooling in cyanoacetic acid and methyl cyanoacetate isolated in low temperature inert gas matrixes

“…Another compound undergoing conformational cooling in matrixes is methyl cyanoacetate (MCA), which we have recently studied [7]. According to the MP2/6-31G** calculations, in the gaseous phase this molecule has two almost isoenergetic conformers differing in the relative orientation of the cyano and methylcarboxyl groups.…”