Knowledge Bank

“The J2$\leftarrow$1andJ3$\leftarrow$2 (Type a) rotational transitions of nine isotopic species of propynal have been observed, yielding the rotational constants B and C. The molecule is planar, and by observing certain Type b transitions in the species HCCCHO and DCCCHO the rotational constant A and hence the inertial defect $\Delta$ could be calculated for these molecules. The value of $\Delta$ in HCCCHO was found to be 0.1715 a.m.u. $A¯2$, and in DCCCHO, 0.1783 a.m.u. $A¯2$. By assuming $\Delta$ to be unaffected by a $C12$ substitution, two independent sets of molecular parameters were calculated. Quite satisfactory agreement was obtained without correcting for centrifugal distortion. The $H1-C1\equiv C2-C3$ chain was found to be bent very slightly at the central carbon away from the oxygen alom. The calculated structural parameters of the molecule are as follows: $H1-C1=1.0551\backslash AAC1C2=1.2088\backslash AA,C2C3=1.4449\backslash AACaO=1.2160\backslash AA,\le C1C2C3=1\circ 42\prime and\le C2C3O=56\circ 18\prime$ No parameters for the aldehyde hydrogen are offered, as substitution at this position has so far not been possible.”