Infrared Electroabsorption Spectroscopy of N,N-Dimethyl-p-nitroaniline in Acetonitrile/C2Cl4: Solvation of the Solute and Self-Association of Acetonitrile

Solvated structures of N,N-dimethyl-p-nitroaniline (DMPNA), an analog of p-nitroaniline (PNA), and self-associated structures of acetonitrile (ACN) in mixed solvents of ACN and C2Cl4 were studied using infrared (IR) electroabsorption and FTIR spectroscopies. IR electroabsorption spectroscopy measures changes in IR absorption intensity upon application of external electric field modulation, which are a sensitive probe for permanent dipole moments. In ACN/CCl4, PNA has been shown to occur as two distinct solvated forms, namely, 1:1 and 1:2 forms, which have one and two ACN molecule(s), respectively, associated with PNA. The IR electroabsorption and FTIR measurements on DMPNA show that, unlike PNA, DMPNA occurs as a monomer in ACN/C2Cl4 rather than as specific solvated structures analogous to the 1:1 and 1:2 forms because of the substitution effect. Not only does the N, N-dimethyl substitution in DMPNA hamper solvation of ACN at the N(CH3)(2) group, but it also indirectly blocks strong interactions with ACN at the NO2 group. Furthermore, by using the AA signal of DMPNA as an internal intensity standard, it was found that the dipole moment of ACN in the DMPNA/ACN/C2Cl4 system is about 1.5 times larger than that of the ACN monomer in dilute CCl4 solution. This large value of the dipole moment in the solution studied here is attributable to the formation of a head-to-tail linear dimer of ACN, whereas the antiparallel dimer is energetically more favorable in the gas phase.