Electron-spin-resonance characterization of nonlinear C4 trapped in solid argon

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Abstract
The electron‐spin‐resonance spectrum of the C4 molecule has been observed by trapping the products of the vacuum ultraviolet photolysis of the diacetylene (C4H2) and 1,3‐butadiene (C4H6) molecules in Ar at 12 K. Using highly dilute samples with argon to parent molecule ratios ranging from 2000:1 to 50 000:1, results in absorption lines with substantially narrower linewidths than those observed previously for C4 produced by the evaporation of graphite. The sharper spectra reveal splitting of the perpendicular lines into separate x and y components, indicating nonlinear character for C4 trapped in solid argon. The possibility that the splitting originates with multiple trapping sites or hyperfine interaction with a hydrogen nucleus is eliminated on the basis of the results of extensive annealing experiments and the lack of any effect when the parent molecules are deuterated. Various carbon‐13 substitutions in the butadiene parent yield hyperfine splitting components for the outer (α) and inner (β) pairs of equivalent carbon nuclei. Measured magnetic constants are gxx=2.0016(5), gyy=2.0041(5), ‖D‖=0.2237(2) cm−1, ‖E‖=2.52(2)×10−4 cm−1, ‖Axx(13Cα)‖=29.7(6) MHz, ‖Ayy(13Cα)‖=29.7(6) MHz, ‖Axx(13Cβ)‖=6.2(6) MHz, and ‖Ayy(13Cβ)‖=7.2(6) MHz. It is concluded that C4 formed by trapping the photolysis products of C4H2 and C4H6 in solid argon is slightly bent.