Knowledge Bank

The infrared vibration-rotation spectrum of $H2CO$ vapor has been measured in the region from 2600 to $3400cm-1$ with resolution ranging from 0.04 to $0.07cm-1$. The use of the Loomis-Wood diagrams and the ASSIGN $diagrams1$ 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 $\nu 3+\nu 6=2719.2 cm-1$.