Structural and conformational properties of 4-Pentyn-1-ol as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectra of 4-pentyn-1-ol, HO(CH2)3C triple bond CH, and one deuterated species (DO(CH2)3C triple bond CH) have been investigated in a Stark spectrometer in the 17.5-80 GHz spectral region at about 0 degrees C, as well as in a pulsed-nozzle Fourier transform spectrometer in the 2.5-14 GHz range. A total of 14 spectroscopically different all-staggered rotameric forms are possible for this compound. It has previously been assumed that a conformer stabilized by intramolecular hydrogen bonding predominates in the gas phase, but the microwave spectrum of this rotamer was not assigned and it is concluded that this form is not present in high concentrations. However, the microwave spectrum indicates that several forms are present, two of which denoted ag+g+ and ag+a were assigned in this work. In these two forms, the H-O-C-C chains of atoms have an antiperiplanar conformation and the O-C-C-C links are synclinal ("gauche"). The C-C-C-C triple bond CH link is synclinal in ag+g+ but antiperiplanar in ag+a. The ag+g+ form is determined to be 1.5(6) kJ/mol more stable than ag+a by relative intensity measurements. The microwave study was augmented by quantum chemical calculations at the MP2/6-311++G** and G3 levels of theory. Both these quantum chemical procedures indicate that there are small energy differences between several rotametric forms, in agreement with the microwave findings. Both methods predict that ag+g+ is the global minimum.