Matrix Isolation Infrared and DFT Study of the Trimethyl Phosphite− Hydrogen Chloride Interaction: Hydrogen Bonding versus Nucleophilic Substitution N. Ramanathan, Bishnu Prasad Kar, K. Sundararajan, and K. S. Viswanathan* ,† Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India *S Supporting Information ABSTRACT: Trimethyl phosphite (TMPhite) and hydrogen chloride (HCl), when separately codeposited in a N 2 matrix, yielded a hydrogen bonded adduct, which was evidenced by shifts in the vibrational frequencies of the TMPhite and HCl submolecules. The structure and energy of the adducts were computed at the B3LYP level using 6-31++G** and aug-cc- pVDZ basis sets. While our computations indicated four minima for the TMPhite−HCl adducts, only one adduct was experimentally identified in the matrix at low temperatures, which interestingly was not the structure corresponding to the global minimum, but was the structure corresponding to the first higher energy local minimum. The Onsager self-consistent reaction field model was used to explain this observation. In an attempt to prepare the hydrogen bonded adduct in the gas phase and then trap it in the matrix, TMPhite and HCl were premixed prior to deposition. However, in these experiments, no hydrogen bonded adduct was observed; on the contrary, TMPhite reacted with HCl to yield CH 3 Cl, following a nucleophilic substitution, a reaction that is apparently frustrated in the matrix. 1. INTRODUCTION The study of hydrogen bonded adducts, both experimental and theoretical, is of considerable interest. 1,2 Studies on hydrogen bonded adducts involving organophosphorus compounds assume significance for various reasons. Organic phosphites and phosphates serve as model systems for understanding biological processes. 3,4 Organophosphorous compounds are also used as extractants in a number of solvent extraction processes. Earlier studies from our group reported the hydrogen bonded adducts of trimethyl phosphate with various proton donors such as H 2 O, C 2 H 2 , and C 6 H 6 . 5−7 The rapid advance of molecular biology owes much to the synthesis of DNA. This synthesis operates by way of phosphites, where the addition of each nucleoside residue is followed by oxidation of the phosphite to phosphate; the resulting phosphates are quite stable under the conditions of the syntheses. 8−12 Although there is a vast body of literature dealing with the chemistry of phosphorus compounds, surprisingly only a few studies are reported for lower valence organophosphorous compounds. They were reported to undergo multistep chemical reactions with complicated kinetic analyses. 12 In short, the kinetic and mechanistic data are sparse for these lower valence organophosphorous compounds. Anderson et al. reported the first gas phase chemical reaction of trimethyl phosphite (TMPhite). 13 They conducted ion− molecule reactions of TMPhite with a variety of nucleophiles such as CH 2 CHCH 2 − , (CH 3 ) 2 CC(CH 3 )CH 2 − , NH 2 − , CH 3 NH − , (CH 3 ) 2 N − , OCH 3 − ,H − , PH 2 − , OH − , and F − . With the first seven nucleophiles, they observed an S N 2 nucleophilic attack, preferentially at phosphorus, which eventually produced the methoxide anion as a leaving group, while the last three nucleophiles reacted preferentially via a carbon attack to produce a dimethyl phosphite anion as the leaving group. In this paper, we have reported the interaction between TMPhite and HCl, both in a low temperature N 2 matrix and in the gas phase. The interaction of TMPhite with HCl in low temperature matrix was found to be different from that of the gas phase. The matrix isolation technique was used to understand the reactivities both in the low temperature matrix and in the gas phase. 2. EXPERIMENTAL SECTION Matrix isolation experiments were carried out using a Leybold AG He-compressor-cooled closed cycle cryostat. The details of the vacuum system and experimental setup are described elsewhere. 14−17 TMPhite (98%, Merck) was used without any further purification. However, the sample was subjected to several freeze−pump−thaw cycles before use. HCl gas was prepared by mixing AR grade H 2 SO 4 and HCl solutions, in a Received: July 13, 2012 Revised: November 2, 2012 Published: November 19, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 12014 dx.doi.org/10.1021/jp306961m | J. Phys. Chem. A 2012, 116, 12014−12023