ROTATIONAL ISOMERIZATION OF ESTERS ON NI(111)

Citation
E. Zahidi et al., ROTATIONAL ISOMERIZATION OF ESTERS ON NI(111), Journal of physical chemistry, 99(51), 1995, pp. 17906-17916
Citations number
78
Categorie Soggetti
Chemistry Physical
ISSN journal
00223654
Volume
99
Issue
51
Year of publication
1995
Pages
17906 - 17916
Database
ISI
SICI code
0022-3654(1995)99:51<17906:RIOEON>2.0.ZU;2-G
Abstract
Reflectance absorbance infrared (RAIRS) and thermal desorption (TPD) d ata for methyl formate and ethyl formate chemisorbed on Ni(111) are fo und to be consistent with surface coverage dependent rotational isomer ization. The most stable gas phase rotameric forms of these molecules are the (Z) or cis forms. However, the reflectance infrared data provi de direct evidence for isomerization to the (E) or anti forms as their surface coverages are increased. This rotational isomerization, aroun d the C-O bond, is observed to occur at coverages less than half that required to saturate the chemisorbed layers. In combination with infra red integrated intensity data, the thermal desorption results indicate that steric repulsion is responsible for the observed rotational isom erization. It is argued that two factors contribute to a reduction of the rotational barrier. First, steric repulsion is reduced continuousl y along the (Z) to (E) rotational coordinate. Second, image dipole sta bilization increases continuously along the (Z) to (E) rotational coor dinate. A number of interesting parallels exist between the chemisorpt ion properties of these three ester molecules on Ni(111) and the prope rties of Lewis acid-carbonyl molecule adducts. These similarities will be discussed briefly with reference to use of Lewis acids to achieve stereoselective control in organic synthesis, and with reference to th e issue of enantioselective heterogeneous catalysis.