CONFORMATIONAL RIGIDITY IN A SELF-ASSEMBLED MONOLAYER OF 4-MERCAPTOBENZOIC ACID ON GOLD

Self-assembled monolayers of 4-mercaptobenzoic acid exhibit infrared s pectral features for the carboxylic acid functionality that are charac teristic of isolated vapor-phase molecules (e.g., sharp O-H stretch ba nd, high-frequency non-hydrogen-bonded carbonyl stretch band) and inco nsistent with molecules that are dimerized or that reside in condensed -phase hydrogen-bonding environments. This is in sharp contrast with t he behavior of omega-mercaptoalkanoic acid monolayers, which exhibit s pectral features characteristic of hydrogen-bonded and/or dimerized ac id groups (e.g., no obvious O-H stretch band, lower frequency carbonyl stretch band). It is postulated that these characteristic differences reflect differing degrees of conformational rigidity in the monolayer s, with the aromatic monolayers being sufficiently rigid and oriented as to prevent the intermolecular hydrogen bonding/dimerization that is commonly observed in aliphatic monolayers.