Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

Abstract

The infrared spectra of 3-pentyn-2-ol, CH 3C CCH(OH)CH 3, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm −1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C 1( a Me). From various bands assigned to a second conformer in which OH is oriented anti to H gem( a H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol −1. The highest energy conformer with OH anti to C 3( a C) was not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a H conformer had slightly lower energy, all the calculations revealed that a Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a Me conformer as more stable by 0.8 and 8.3 kJ mol −1 relative to a H an a C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.