SPIN SPLITTINGS AND ROTATIONAL STRUCTURE IN THE LOWEST TRIPLET STATE OF FORMALDEHYDE$^{\ast}$

$^{\ast}$ Supported in part by the Army Research Office and the National Science Foundation. $^{1}$ G. W. Robinson and V. E. DiGiorgio, Can. J. Chem. S6, 31 (1958); J. Chem. Phys. S1, 1678 (1959). $^{2}$ W. T. Raynes, This Symposium (1963). $^{3}$ J. H. Van Vleck, Rev. Mod. Phys. 23, 213 (1951). $^{4}$ J. Hougen, Nat. Res. Council Canada, (private communication). $^{5}$ J. W. Sidman, J. Chem. Phys. 29, 644 (1958). $^{6}$ A. D. Walsh, J. Chem. Soc. 2306 (1953).Author Institution: Gates and Crellin Laboratories of Chemistry, California Institute of TechnologyThe formaldehyde molecule is one of the simplest polyatomic molecules for which triplet-singlet bands possessing well-resolved rotational structure have been $photographed^{1}$. A rotational analysis of the spectrum therefore provides an excellent testing ground for current theories of (a) the effects of spin-rotation, spin-orbit, and spin-spin interactions on the rotational levels of nonlinear molecules in triplet states and (b) the relative intensities of the various branches of triplet$\leftarrow$singlet bands. Using the observed frequencies of Robinson and DiGiorgio, a preliminary $analysis^{1, 2}$ of the $0^{+}$ and $1^{+}$ bands of the $^{3}A_{2}\longleftarrow ^{t}A_{1}$ transition has been extended. The major results of the analysis are: (1) The rotational structure is well-described by Van Vleck's $theory^{3}$. In particular the small effect of molecular asymmetry on the spin splittings has been detected and explained quantitatively in terms of this theory; (2) the branch intensities agree with those predicted by $Hougen^{4}$. In particular the absence of transitions to the $F_{2}$ component of the triplet state can be understood in the light of this theory; (3) the cause of the spin-forbidden transition is found to be a spin-orbit coupling involving the $^{1}A_{1}$ state at 8 e. v. in agreement with the theoretical deductions of $Sidman^{5}$; (4) the molecule is nonplanar in the triplet state as predicted by $Walsh^{6}$. The following geometrical parameters have been obtained for the $0^{+}$ inversion level: $r(C-H) = 1.08$ {\AA} (assumed); $r(C-O) = 1.29$ {\AA}; $< HCH = 120^{\circ}$; out-of-plane angle $= 40^{\circ}$. Values of the rotational constants, distortion constants and spin-interaction constants will be given. Curves depicting spin splittings in the triplet state will also be presented