Ammonia dimerization

Microwave studies in molecular beams are usually limited to studying the ground vibrational state of the complex. For complexes made up of two molecules (as opposed to atoms), the intennolecular vibrations are usually of relatively low amplitude (though there are some notable exceptions to this, such as the ammonia dimer). Under these circumstances, the methods of classical microwave spectroscopy can be used to detennine the stmcture of the complex. The principal quantities obtained from a microwave spectmm are the rotational constants of the complex, which are conventionally designated A, B and C in decreasing order of magnitude there is one rotational constant 5 for a linear complex, two constants (A and B or B and C) for a complex that is a symmetric top and tliree constants (A, B and C) for an... 

Zhu, T., and W. Yang. 1994. Structure of the Ammonia dimer Studied by Density Functional Theory. Int. J. Quant. Chem. 49, 613. 

Kieninger, M., and S. Suhai. 1996. Conformational and Energetic Properties of the Ammonia Dimer-Comparison of Post-Hartree-Fock and Density Functional Methods. J. Comp. Chem. 17, 1508. 

As noted above, more recent experimental studies of the ammonia dimer favor the modified cyclic structure 30. In light of these latest findings, new parameters have been developed [specifically, the VdW radius of H(N) was increased to 1.6 A, similar to that of H(O) and H(C)] resulting in interaction energies of —2.17 and —0.56 kcalmol-1 and N... N distances of 3.07 and 2.50 A for the linear and modified cyclic structures, respectively. 

As of this writing, there is some controversy about the gas phase structure of the ammonia dimer (Baum, R. M., Chem. Eng. News, 1992, October 19, 20-23). The traditional view of the dimer, containing a linear hydrogen bond, which is supported by most theoretical and experimental studies, has been question by Klemperer and co-workers on the basis of microwave measurements (Klemperer, W. Nelson Jr., D. D. Fraser, G. T., Science, 1987, 238, 1670). 

Figure 8.20. The structure of ammonia dimer. The centers of mass of the two subunits lie on the z axis. 0, and 02 are the angles between the C3 axes of the subunits and the z axis.

Symmetry of Rotational Wave Functions for trans and cis Paths of Ammonia Dimer Interconversion... 

Zhu T, Yang W (1994) Structure of the ammonia dimer studied by density functional theory, Int J Quant Chem, 49 613—623... 

One can conclude that in many cases, an analysis of the electrostatic interaction between a pair of molecules will provide a very reasonable estimate of the angular aspects of their interaction. Exceptions will likely occur in the case of very flat surfaces, as in the case of the ammonia dimer, or in weak complexes where dispersion plays an increasingly important role. 

Perhaps more than any other complex, the ammonia dimer has provided the most intriguing puzzle in piecing together its equilibrium geometry. It was presumed early on that this dimer would form a H-bond much like the other small hydrides such as HF and H O, even if perhaps somewhat weaker. Indeed, there were indications from experimental work that the equilibrium structure was in fact linear. For that reason, most of the early ab initio calculations focused on the linear type of structure. 

Table 2.29 Calculated properties of linear geometry of ammonia dimer at the SCF level. Data all energies in kcal/mol.

Figure 2.1 I Three candidate equilibrium geometries of the ammonia dimer.

Cybulski considered the ammonia dimer using even larger basis sets, as many as 200 functions, also with bond functions included. He noted a delicate balance between the specific functions used, or the centers of the bond functions, and the relative stability of the linear and cyclic geometries and warns against using bond functions without first carefully... 

Table 2.30 Variation of interaction energy (—of ammonia dimer upon level of correlation. Data, in kcal/mol, were calculated with [753/41] basis set, augmented by bond functions. Both geometries fully optimized at MP2 level, including BSSE corrections ". ...

Latajka, Z. and Scheiner, S., Effects of basis set and electron correlation on the calculated properties of the ammonia dimer, J. Chem. Phys. 81, 407-409 (1984). 

Perchard, J.-R, Bohn, R. B., and Andrews, L., Matrix infrared and Raman spectra of the inequivalent submolecules in the ammonia dimer, J. Phys. Chem. 95, 2707-2712 (1991). 

Hassett, D. M., Marsden, C. J., and Smith, B. J., The ammonia dimer potential energy surface Resolution of the apparent discrepancy between theory and experiment , Chem. Phys. Lett. 183, 449-456 (1991). 

Cybulski, S. M., Extended basis set calculations of the interaction energy and properties of the ammonia dimer, Chem. Phys. Lett. 228,451-457 (1994). 

Muguet, F. F. and Robinson, G. W., Towards a new correction method for the basis set superposition error Application to the ammonia dimer, J. Chem. Phys. 102, 3648-3654 (1995). 

Olthof, E. H. T., van der Avoird, A, and Wormer, R E. S., Structure, internal mobility, and spectrum of the ammonia dimer Calculation of the vibration-rotation tunneling states, J. Chem. Phys. 101, 8430-8442 (1994). 

Table 3.52 Frequencies (cm ) and intensities (D A amu ) calculated for the ammonia dimer in its linear configuration. ...

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