Infrared and Raman Spectra, Conformational Stability, Ab Initio Calculations, and Vibrational Assignment of 1-Penten-4-yne

Abstract

The infrared spectra (3500-50 cm -1 ) of gas and solid and the Raman spectra (3500-50 cm -1 ) of liquid and solid 1-penten-4-yne, CH 2 dCHCH 2 CtCH, have been recorded. Variable-temperature studies over the range -105 to -150 °C of the infrared spectra (3500-400 cm -1 ) of the sample dissolved in liquid krypton have also been recorded. By utilizing the relative intensities of five syn/gauche conformer pairs, the syn conformer is found to be the lower energy form with an enthalpy difference of 248 ( 25 cm -1 (2.97 ( 0.30 kJ/mol). At ambient temperature it is estimated that there is 38% ( 2% of the gauche conformer present. Equilibrium geometries and total energies of the two conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory (DFT) calculation with the B3LYP method using a number of basis sets. The smaller basis sets with diffuse functions predict the gauche conformer to be the more stable form, whereas other calculations predict the syn conformer to be the lower energy rotamer. The B3LYP calculations predict a much lower energy difference between the conformers than found experimentally. A complete vibrational assignment is proposed for the syn conformer and many of the fundamentals have been identified for the gauche form on the basis of the force constants, relative intensities, and depolarization ratios obtained from MP2/6-31(d) ab initio calculations as well as on rotational-vibrational band contours obtained from the predicted equilibrium geometric parameters. The r 0 structural parameters are estimated for 1-penten-4-yne from ab initio MP2/6-311+G(d,p) calculations, and comparisons are made with those of allyl fluoride and cyanide. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.

Infrared and Raman spectra, conformational stability, ab initio calculations and vibrational assignment of 1-hexen-4-yne

“…The most interesting part of the spectrum is the far infrared region where the C= =CC in-plane and out-of-plane bends as well as the asymmetric torsion are found. Similar to those of 2-hexyne, 26 2-pentyne 27 and 1-penten-4-yne, 16 the predicted frequency of the C 3 C 4 C 10 out-of-plane bend (n 33 ) from MP2/6-31G(d) calculations is always lower than the observed frequency. Therefore, the n 33 fundamental of the cis conformer is assigned to lines at 368 and 373 cm À1 in the Raman spectra of the liquid and the polycrystalline solid, respectively, which is about 100 cm À1 higher than the ab initio predicted value.…”

“…However, 3-cyanopropene, 7 CH 2 =CHCH 2 CN, where the experimental enthalpy difference is 336 cm À1 , shows that the cis conformer is the more stable form. For another similar molecule, 1-penten-4-yne, 16 the MP2/6-311+G(d, p) calculations predict the gauche form to be more stable by 13 cm À1 which is not consistent with the experimental result of an enthalpy difference of 248 cm À1 with the cis rotamer being more stable. However by utilizing a larger basis set 6-311+G(2d, 2p), a much larger value of 183 cm À1 is determined but now with the cis conformer being more stable.…”

“…It is interesting to compare the structural parameters between 1-hexen-4-yne and 1-penten-4-yne 16 where the only difference is the substitution of the methyl group of the former molecule with a hydrogen atom. The structural parameters are essentially the same for the two molecules according to the ab initio MP2/6-31G(d) calculations.…”

“…These near degenerate fundamentals shift and split as the two CH 3 deformations at 1441 and 1436 cm À1 in the infrared spectrum of the polycrystalline solid. In the study of another similar molecule, 1-penten-4-yne, 16 the CH 2 deformation, wag and twist fundamentals of the cis conformer were assigned at 1400, 1324 and 1208 cm À1 , respectively, in the infrared spectrum of the solid. Because of the similarity between these two molecules and by using the '' group frequency '' principle, the corresponding fundamentals of cis 1-hexen-4-yne are assigned at 1399, 1324 and 1212 cm À1 , respectively, in the infrared spectrum of the solid.…”

“…The structural parameters, conformational stability, and vibrational spectra of the 3-monosubstituted propenes, CH 2 = CHCH 2 X, where X ¼ F, 1,2 Cl, 3 Br, 4,5 I, 6 CN, 7 CH 3 , 8 SiH 3 , 9 NH 2 , [10][11][12][13] OH, 14,15 C= =CH, 16 etc. have been of interest for many years.…”