UV-induced cyclization in myrcene isolated in rigid argon environment: FT-IR and DFT study

The UV-induced photochemistry of monomeric myrcene [CH2C(R)–C(H)CH2, where R is –CH2CH2CHC(CH3)2] isolated in a low-temperature argon matrix (13 K) was investigated by infrared spectroscopy and quantum-chemical calculations. Twenty-seven unique conformers of myrcene were predicted theoretically at both the DFT(B3LYP) and MP2 levels of approximation, with the 6-311++G(d,p) basis set. The most stable conformers were found to have the vinyl fragment in the s-trans orientation, and dominate in the equilibrium conformational mixture at room temperature. The experimental FTIR spectrum of the matrix-isolated compound was found to be well-reproduced by the superposition of the spectra of the three most stable forms of myrcene. These three forms differ structurally only by the arrangement of the vinyl groups relatively to the side chain. Narrowband selective as well as broadband UV irradiation of the matrix-isolated compound were found to lead to its cyclization to the cyclobutene-type product, 1-(4-methylpent-3-en-1-yl)cyclobutene. By analogy to butadiene, this photoprocess is proposed to result from accessing a conical intersection between the πσ* S1 “dark” and S0 states.