The molecular structure of acyclic aralkyl compounds studied by a crystallographic database survey. Relevance of the intramolecular CH/π hydrogen bond to conformation

Abstract

A database study was carried out, by the use of the Cambridge Structural Database, to investigate the conformation of aralkyl compounds ArCH₂XCH₂Y 1 and ArCHCH₃XCH 2 in crystals. The structure bearing R (R: any group) and Ar in the syn relationship has often been found in these compounds. The proportion of crystal structures bearing R and Ar in the syn relationship relative to the anti conformation (r_syn/anti) varied from 0.55 for 1 to 3.68 for 2. The logarithm of r_syn/anti was plotted against the difference in Gibbs energy ΔG_{syn − anti} obtained by MO calculations of model compounds at the MP2/6-311G(d,p)//MP2/6-31G(d) level: C₆H₅CH₂XCH₃ and C₆H₅CHCH₃XCH₃. A linear correlation has been shown between lnr_syn/anti and ΔG_{syn − anti}. The CH/π interaction is suggested to operate in controlling the R/Ar-folded crystal structure of these aralkyl compounds.