Microwave spectra and structures of 1-cyano- and 1-iodo-adamantane

Abstract

The microwave spectra of three isotopic species of 1-cyanoadamantane (¹²C¹⁴N, ¹³C¹⁴N and ¹²C¹⁵N) and of the common isotopic species of 1-iodoadamantane are characteristic of a rigid symmetric rotor with ground state rotational constants B= 852.213, 843.773, 834.171 and 421.700 MHz respectively. Rotational constants for several quanta of a low frequency vibration for each species of 1-cyanoadamantane are also recorded.

The moment of inertia of the adamantyl radical referred to its own principal axis system, I^*_rad, is derived from observed ground state rotational constants for 1-cyanoadamantane and on comparison with the corresponding quantity previously derived for the 1-chloro- and 1-bromo-compounds shows that the nature of the substituent does not seriously affect the adamantyl cage structure. If it can be assumed that all r(C—C) are equal and all interbond angles are tetrahedral as in adamantane itself and further if r(C—H)= 1.09 Å, the following bond lengths result: r(C—C)= 1.543 ± 0.001 Å, r(C—CN)= 1.466 ± 0.003 Å. For 1-iodoadamantane, r(C—I)= 2.160 ± 0.002 Å if in addition r(C—C) is assumed.

A comparison of r(C—X) values from microwave spectroscopy for methyl, ethyl, t-butyl and adamantyl compounds for X = F, Cl, Br, I and CN indicates an anomaly for 1-fluoroadamantane which can be explained if the adamantyl cage is therein distorted at the bridgehead relative to the other compounds.