The infrared spectra of o-hydroxyphenylalkyl ketones; o-hydroxyacetophenone (OHAP), ohydroxypropiophenone (OHPP), o-hydroxybutyrophenone (OHAP), o-hydroxyisobutyrophenone (OHIBP) and w-bromo-o-hydroxyacetophenone (w-BrOHAP) have been recorded in the solid, liquid and vapour phases in the region 4000-200 cm-'. The results for the three phases show that two stable rotational conformers exist in each of OHAP and w-BrOHAP and more than two in each of the others. They are cis and tram in the case of OHAP and w-BrOHAP, whilst ch and tracts are the majority species in OHPP, OHBP and OHIHP. Vibrational frequency assignments of some of the fundamental modes for the common ci.y and truns conformers are made. Enthalpy differences (AH") between them were determined and used as a measure of stability of the conformers for each compound in the three phases. It has been found that the cis conformer is the predominant form in the vapour phase in all these compounds. This form is also the more stable in OHBP and OHIBP liquids and in w-BrOHAP solid. IT HAS BEEN shown by means of i.r. spectroscopy that o-halogeno, o-nitro and omethylacetophenone exist as two rotational conformers [ 1,2]. o-Hydroxyacetophenone is reported to exist in the chelated form as rigid molecules [3-51. Studies on acetophenone substituted on the methyl group by halogen atoms suggest the presence of cis and gauche conformers associated with hindered rotation about the CO-CH7X linkage [6,7]. Further, i.r. studies on o-hydroxyacetophenone and its derivatives with substituents on the benzene ring show the presence of cLs and truns conformers [8-121. However, evidence for the conformations in all these cases was deduced from the observed splittings on carbonyl bands. NMR studies have been published on hydroxyacetophenone with emphasis on conformational preferences in solution [13-211. It seems that it is not yet possible to conclude that the different conformations of o-hydroxyacetophenone are due to restricted rotation. Therefore, more extensive studies on the conformation in o-hydroxyacetophenone and its derivatives are required and are reported in the present work. This work has involved the i.r. investigation of several o-hydroxphenylalkyl ketones; o-hydroxyacetophenone (OHAP), o-hydroxypropiophenone (OHPP), ohydroxybutyrophenone (OHBP), o-hydroxyisobutyrophenone (OHIBP) and w-bromo o-hydroxyacetophenone
(w-BrOHAP) in the solid, liquid and vapour phases. The objective is to investigate the possible existence of conformations in these molecules in every phase. The various factors influencing the relative stabilities of the conformers will be investigated in addition to determination of Ati values between them. To this end, vibrational frequency assignments of some of the fundamental modes have been made for each conformer. To the best of our knowledge, no previous i.r. work has been published on these compounds neither collectively nor individually.
EXPERIMENTAL
OHAP
is commercially available and was obtained from Fluka AG Fabrik (pract. 90-95% stated purity). OHPP, OHBP and OHIBP were prepared according to the Fries method [23], and w-BrOHAP was prepared according to Ref. [24]. They are all liquids and have been purified by fractional distillation three tinies under reduced pressure. Middle portions were collected. They