Bond angle from rotational spectra

Interatomic distances as calculated from the analysis of the rotational fine structure of the ultraviolet spectrum are C-C, 1.395 A C-N, 1.341A and C-H, 1.085 A.66 These are very similar to the bond lengths for pyridine which are C-2-C-3, 1.3945 A C-3-C-4, 1.3944 A and C-2-N, 1.3402 A. The C-N-C bond angle in pyrazine is 115° and the C-C-N bond angle 122.5°.56,67 A delocalization energy for pyrazine of ca. 18 kcal/mole is indicated from heats of combustion data.68 The C=N bond energy in 2,2,5,5-tetramethyl-2,5-dihydropyrazine has been calculated to be 130.3 kcal.58a... 

The principal application of the Kraitchman equations [Eq. (9)1 is for the determination of the atomic coordinates, at, bSi and cs. From a study of the rotational spectrum of the parent and of a species with single isotopic substitution the coordinates of the substituted atom may be determined. These coordinates are referred to as substitution coordinates or rs coordinates. Each new species yields new coordinates, and since all of the coordinates are in the same coordinate system, the calculation of substitution or rs bond distances and bond angles is a simple process. Costain,s demonstrated that there are definite advantages to the use of the Kraitchman equations to obtain molecular parameters. These advantages are sufficient to make the use of Kraitchman s equations the preferred method of structure determination from ground-state rotational constants. 

What information about the bond lengths and angles of the following molecules could be determined from spectroscopic measurements In which cases would you expect to observe a pure rotation spectrum ... 

The centrifugal force produced by rotation distorts the molecule from its equilibrium configuration and the bond distance and angles change sUghtly. Hence, the rotational spectrum is no longer characterized by a set of equiUbrium moments of inertia. Additional terms in the Hamilitonian are required to account adequately for the observed... 

The //NMR spectrum (Fig. 2.19) displays anAB system for the protons adjacent to this bond the coupling constant = 72 Hz. From this can be deduced first that the dihedral angle 9 between the C7/bonds is about 180°, second that conformer 14b with minimised steric repulsion between the substituents predominates and third that there is restricted rotation around this CC bond. 

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