Quantum-Chemical Investigation of Structure and Vibration Spectrum of [C(NPCl3)3]+ Cation

Abstract

Structural parameters of the cation [C(NPCl 3 ) 3 ] + in vacuum and in acetonitrile are calculated by the methods RHF/6-311G(3d 6 ), RHF/6-31(3d 5 ) and B3LYP/6-311(3d 5 f 7 ). Formation energy of the free adduct (MeCN) 2[C(NPCl 3 ) 3 ] + is calculated and nonspecific character of interaction of the cation with liquid acetonitrile is established. Vibration spectrum of the cation is calculated and theoretical interpretation of IR and Raman spectra of the salt [C(NPCl 3 ) 3 ] + [SbCl 6 ] ! is refined. Stable tris(trichlorophosphoranylidenamino)carbenium cation (I) is a component of the soluble in acetonitrile salt [C(NPCl 3 ) 3 ] + [SbCl 6 ] ! (II) which can be obtained from [C(NH 2 ) 3 ] + [SbCl 6 ] ! and PCl 5 [1] or from [C(N 3 ) 3 ] + [SbCl 6 ] ! and PCl 3 [2]. It was es-tablished by X-ray structural investigation that in the crystal of II the cation I has almost symmetrical (C 3h ) structure with average bond lengths C3N 135, N3P 155, and P3Cl 196 pm [2,3].In this work we applied RHF/6-311G(3d 6 ), RHF/6-31(3d 5 ) and DFT B3LYP/6-311(3d 5 f 7 ) methods of GAUSSIAN [4] package for quantum-chemical investigation of structure of the cation I and of (MeCN) 2 (I) adduct both free (vac) and dissolved in acetonitrile (solv) (see Fig. 1). The nuclei of C, H, P and of three Cl atoms all fall to the symmetry plane xy of the cation I. Symmetry axes of acetonitrile moiety in the (MeCN) 2 (I) adduct coincide with the z axis. To account for the solvent effect (acetonitrile) we applied polarizable continuum model (C-PCM). (Table 1) depend only slightly on the used basis and the solvent reaction field value. Acetonitrile molecules contributing to (MeCN) 2 (I) solvatocomplex also show scarce effect. The C3N bond length is closer to that of C=N in methylimine (MeNCH 2 , 130 pm) rather than C3N in amines (Me 3 N 145 pm, Me 2 NH 146 pm, and MeNH 2 147 pm) [5]. Calculated bond lengths and bond anglesWe calculated IR spectrum of the cation I in the medium of acetonitrile by the method DFT B3LYP/6-311G(3d 5 f 7 ) without adjusting coefficients. Wave numbers and integral intensities of IR absorption are affected by solvent (Table 2), but the modes of normal vibration (Table 3) are insensitive.Comparing experimental IR and Raman spectra of the salt II with the spectra of related compounds, Mueller [3] noted extremely low frequency of nonsymmetrical bond vibration of phosphazene bonds which could not be explained satisfactorily and in his opinion it conforms hardly to experimental bond lengths. However, our calculation satisfactorily obeys both experimental bond lengths and vibration wave numbers of cation I. The maxima at 1003 and 1016 cm !1 in the IR spectrum or 1002 and 1013 cm !1 in the Raman spectrum are induced by vibration of the whole atomic group CN 3 P 3 rather than by assumed in Fig. 1. Equilibrium configuration of nuclei in the cation I(solv).