Singlet−Triplet Gap, and the Electronic and Vibrational Spectra of Chlorophenylcarbene:  A Combined Theoretical and Experimental Study

Abstract

Minimum energy structures of singlet and triplet chlorophenylcarbene, a prototypical carbene, were computed. The singlet-triplet energy separation was predicted to be 7.84 and 7.70 kcal/mol at the UCCSD(T)/6-31+G* and QCISD(T)/6-31+G** levels of theory, respectively, after zero-point correction. This is slightly larger than that predicted by the CAS(6,6) (4.5 kcal/mol), local spin density approximation (5.6 kcal/mol), and the BLYP (7.3 kcal/mol) methods with the 6-31G* basis set reported by Trindle et al. The UV-vis and IR spectra of chlorophenylcarbene were analyzed with the aid of the CASPT2/CASSCF(10,10) and the B3LYP/ 6-31G* levels of theory, respectively. The UV-vis and IR spectra of chlorophenylcarbene were assigned on the basis of these calculations. The ab initio calculations predicted the existence of strong absorption bands in the UV and a weak band in the visible in good agreement with published spectra. The long (750 nm) wavelength band corresponds to electron promotion from the lone pair σ (HOMO) to the π* (LUMO). On the basis of the calculated harmonic frequencies, we cannot assign the 1244 and 1600 cm -1 IR bands observed in an argon matrix to chlorophenyl carbene. The most intense IR band (1225 cm -1 ) corresponds to the symmetric C-C stretch of the carbene and aromatic carbon. The asymmetric and symmetric C-C-Cl stretches are assigned to the bands observed at 847 and 739 cm -1 , respectively.

Solvation and complexation of carbenes – a perspective

“…, with strong absorption at 324 nm and weaker absorptions at 484 and 596 nm . PhCCl in pentane has a strong π → p absorption at 308 nm and a much weaker σ → p absorption near 700 nm . With this information, and computational analysis (see the following text), we assign the strong absorption at 324 nm mainly to PhCCl and the weaker absorption at 596 nm partly to PhCCl.…”

“…(a)). In the case of PhCCl, the strong π → p absorption at 308 nm is accompanied by a much weaker σ → p absorption near 700 nm . With an alkylchlorocarbene, for example, MeCCl, the π → p absorption is absent; only the weak σ → p absorption remains, Fig.…”

Matrix and Time-Resolved Infrared Spectroscopy of Chloro-p-nitrophenylcarbene and Related Species

“…This band is essentially a HOMO-LUMO transition in which an electron is promoted from the in-plane hybrid orbital to the out-of-plane p orbital. 12 Time-Resolved Infrared Spectroscopy. The IR band of 1 at 1206 cm -1 is attractive for TRIR spectroscopy because it is an intense band unique to the carbene system.…”

Visualizing Carbene Equilibria

“…Following the direct measurement of K for the PhCCl/PhCCl 2 − system, we studied the equilibria between a series of ring‐substituted ArCCl and ArCCl 2 − ; cf. Eq.…”