Rotational spectra of o-, m-, and p-cyanophenol and internal rotation of p-cyanophenol.

Rotational spectra of p-, m-, and o-cyanophenol have been measured in the range of 10.5-21 GHz and fit using Watson's A-reduction Hamiltonian coupled with nuclear quadrupole coupling interaction terms for the (14)N nuclei. Ab initio calculations at the MP2/6-311++G(d,p) and CCSD(T)/6-311++G(d,p) levels predict the cis conformers of m- and o-cyanophenol to be more stable than the corresponding trans conformers. A natural bond orbital analysis of the hydrogen bonding interaction in o- and m-cyanophenol revealed an intramolecular hydrogen bond that preferentially stabilizes the cis conformer of o-cyanophenol but there was no evidence of hydrogen bonding interactions in cis m-cyanophenol. We recorded 25 a- and b-type rotational transitions for cis o-cyanophenol; the rotational constants are A = 3053.758(2) MHz, B = 1511.2760(3) MHz, and C = 1010.7989(2) MHz. The trans conformer of o-cyanophenol was not observed. We recorded 14 a- and b-type rotational transitions for cis m-cyanophenol and 16 a- and b-type rotational transitions for trans m-cyanophenol. The rotational constants are A = 3408.9200(2) MHz, B = 1205.8269(2) MHz, and C = 890.6672(1) MHz and A = 3403.1196(3) MHz, B = 1208.4903(2) MHz, and C = 891.7241(2) MHz for the cis and trans species, respectively. Rotational transitions of the p-cyanophenol monomer are split due to the internal rotation of the hydroxyl group with respect to the aromatic ring. We recorded 25 a- and b-type rotational transitions for p-cyanophenol; the b-type transitions are split by 40 MHz. The rotational constants are A = 5612.96(2) MHz, B = 990.4283(6) MHz, and C = 841.9363(6) MHz. The ground state spitting DeltaE is 20.1608(6) MHz and the barrier to internal rotation, V(2), is 1413(2) cm(-1) from a fit of the rotational transitions to an internal axis system Hamiltonian. The barrier to internal rotation was modeled at the MP2/6-311++G(d,p) level and the effects of substituents on the phenolic ring and the barriers to internal rotation are discussed.