VIBRATION-ROTATION SPECTRUM OF $H_{2}CO$ IN THE C-H STRETCHING BAND REGION

Abstract

The infrared vibration-rotation spectrum of $H_{2}CO$ vapor has been measured in the region from 2600 to $3400 cm^{-1}$ with resolution ranging from 0.04 to $0.07 cm^{-1}$. The use of the Loomis-Wood diagrams and the ASSIGN $diagrams^{1}$ has led to an extensive assignment of the rotational structure of the $\nu_{1}$ and $\nu_{5}$ hands. A large number of K levels of both the $\nu_{1}$ and $\nu_{5}$ bands are perturbed by the combination bands $\nu_{3} + \nu_{6}, \nu_{2} + \nu_{4}$, and $\nu_{2} + \nu_{6}$ whereas the c-type Coriolis interaction between $\nu_{1}$ and $\nu_{5}$ is weak. The following effective molecular constants (in $cm^{-1}$) are determined: \begin {eqnarray*} \nu_{1} = 2782.49(1), \tilde{A}_{1} = 9.250(5), \tilde{B}_{1} = 1.2968(6), \tilde{C}_{1}= 1.1321(2), \\ \nu_{5} = 2843.35(2), \tilde{A}_{5} = 9.224(2) \tilde{B}_{5} = 1.2936(2), \tilde{C}_{5}= 1.1303(1),\\ \end {eqnarray*} where the numbers given in parentheses are three times the standard errors in the last digits. The perturbing levels of the combination bands $\nu_{2} + \nu_{4}$ and $\nu_{3} + \nu_{6}$ have been located by the analysis of the accidental perturbations in $\nu_{1}$ and $\nu_{5}$, and the structure of the latter band has been assigned. The band origins are estimated to be $\nu_{2} + \nu_{4} = 2906.0 \ cm^{-1}$ and $\nu_{3} + \nu_{6} = 2719.2 \ cm^{-1}$.