ESR spectrum and structure of the n -propyl radical

Abstract

We have observed the electron spin resonance (ESR) spectrum of the n -propyl radical in argon at 4 °K, following its production by the photolysis at 2537 Å of n -propyl iodide in the argon matrix. The spectrum is much better resolved than previously observed spectra of this radical, and thus could be analyzed in detail to obtain the following complete set of magnetic constants for the radical: g1 = 2.00290, g2 = 2.00302, g3 = 2.00249; A1(α) = −28.7. A2(α) = −17.8, A3(α) = −21.0 Oe; and A1(β) = 37.7, A2(β) = 33.3, A3(β) = 33.9 Oe. Axis 1 is the CβCα bond, and the plane determined by axes 1 and 2 approximately bisects the β-CH2 bond angle. Comparison of the observed hyperfine structure (hfs) constants with calculated values shows that the radical is rapidly reorienting between approximately equivalent conformations in which the α-CH2 plane makes angles of approximately ± 30 ° with the 1–2 plane of the principal axes of the magnetic tensors. However, a small triplet splitting (5 Oe) of the center line of the spectrum indicates that the β protons, and thus the aforementioned conformations of the radical, are not exactly equivalent. The outer lines of this triplet can be accounted for if certain radicals are reorienting either between a pair of conformations in which the α-CH2 plane makes angles of 35 ° and − 15 ° with the 1–2 plane (state I) or between the pair of conformations with angles of 15 ° and − 35° (state II), which pairs of conformations are equivalent except for an interchange of the β-proton hfs constants. The center line of the triplet can be explained if either some but not all of the radicals occupy matrix sites which permit rapid reorientation between states I and II, or if there is tunnelling between states I and II at a frequency of the same order of magnitude as the difference between the β-proton hfs constants in a single state. As a sidelight to this study, we observed the ESR spectrum of the ethyl radical. The resulting hfs constants for ethyl are in good agreement with those obtained by McDowell et al., and, in addition, we obtained the following components of the g factor tensor: g∥ = 2.00268, and g⊥ = 2.00273, where the parallel axis is the CC bond.