35-Chlorine NQR spectra and molecular structure of solid chlorophosphoric compounds in the series of 1,3,2 λ5-Benzodioxaphosphole

Abstract

Nuclear quadrupole resonance (NQR) is a high-resolution spectroscopic method of molecular structure determination for solids due to its capability to give signals at specific frequencies for each resonant nucleus in a molecular structure. 35-Chlorine NQR is used here to establish the molecular structure of a number of crystalline chlorine-containing compounds of pentavalent phosphorus and to study their coordination isomerism, which is related to the chlorotropy effect. The 35C1 NQR spectra of solid 4,5-CI2C6H202PCI 3 (I), C6H402PCI2N=CCICCI 3 (II), and C6H402PCI= NCC12CF 3 (HI) (Table 1) were recorded at 77 K on a pulse NQR spectrometer (resonance frequency measurement error v up to _.+5 kHz, nuclear quadrupole spin-lattice relaxation time T 1 about ___10%). The results of measurements in Table i show how 35C1 NQR lines are grouped in the compounds under study. Figure 1 demonstrates the spectrograms of these compounds and compares them with the literature data for "reference" compounds, making it possible to assign the resonance frequencies to molecular fragments in crystals I-III and thus to identify the molecular structure of the corresponding compounds. It is natural to compare the 35-chlorine resonance frequencies of P-CI bonds in sample I with the 35C1 NQR spectrum of the phosphorane C6H402PC13 (spectrumA, Fig. 1). In the latter spectrum, the two high-frequency resonance signals belong to the equatorial chlorines at phosphorus, and the low-frequency signal corresponds to the axial chlorine [1]. The doubled number of 35C1 NQR lines with respect to the number of chlorine atoms in molecule I for both P-CI and C--C1 bonds indicates the presence of two nonequivalent molecular sites in the unit cell of the crystal. The 35C1 NQR lines of P-C1 bonds for the nonequivalent molecules in this double set may be grouped as follows (Table 1): lines 5, 6, and 9 one molecule; lines 7, 8, and 10 the other (according to characteristic ranges of 35-chlorine resonance frequencies for CI e (equatorial) and CI a (axial) and taking into account the splittings and the ratios of the times T 1 in the NQR spectra of the nonequivalent molecules). As to 35C1 NQR lines of C-CI bonds, their assignment to a position in the benzene ring is quite clear, since the 35C1 NQR spectrum of 1,2,4,5-tetrachlorobenzene [2] (spectrum B, Fig. 1) is observed in the same frequency range. Compounds II and III can exist as forms with pentaand tetracoordinated phosphorus atom (chiorotropic tautomerism in the PNC triad):