Here’s another fine paper from the Alonso group employing laser ablation molecular beam Fourier transform microwave spectroscopy coupled with computation to discern molecular structure. In this work they examine the low-energy tautomers of guanine.1 The four lowest energy guanine tautomers are shown in Figure 1. (Unfortunately, Alonso does not include the optimized coordinates of these structures in the supporting information – we need to more vigorously police this during the review process!) These tautomers are predicted to be very close in energy (MP2/6-311++G(d,p), and so one might expect to see multiple signals in the microwave originating from all four tautomers. In fact, they discern all four, and the agreement between the computed and experimental rotational constants are excellent (Table 1), especially if one applies a scaling factor of 1.004. Once again, this group shows the power of combined experiment and computations!

Figure 1. Four lowest energy (kcal mol^-1, MP2/6-311++G(d,p)) tautomers of guanine.

Table 1. Experimental and computed rotational constants (MHz) of the four guanine tautomers.

References

(1) Alonso, J. L.; Peña, I.; López, J. C.; Vaquero, V., "Rotational Spectral Signatures of Four Tautomers of Guanine," Angew. Chem. Int. Ed. 2009, 48, 6141-6143, DOI: 10.1002/anie.200901462

InChIs

Guanine: InChI=1/C5H5N5O/c6-5-9-3-2(4(11)10-5)7-1-8-3/h1H,(H4,6,7,8,9,10,11)/f/h8,10H,6H2
InChIKey=UYTPUPDQBNUYGX-GSQBSFCVCX

Yet more on the benzene dimer. Lesczynski has optimized 9 different benzene dimer configurations, shown in Scheme 1.¹ There are two T-shaped isomers, where a hydrogen from one benzene interacts with the center of the π-cloud of the second. There are two bent versions of the T-shape, called Bent-T-shape. There are two sandwich configurations and two variants where the benzenes are parallel but displaced. Lastly, they report on a new variant, the V-shape configuration. (Once again, the author has not deposited the structures and so I can’t produce interactive figures!)

Scheme 1

The structures were optimized at MP2/aug-cc-pVDZ and then single point energies computed at MP4(SDTQ)/aug-cc-pVDZ and corrected for basis set superposition error. I list these energies in Table 1. They authors note that in comparison with CCSD(T) computations one has to adjust the amount of BSSE correction – which just supports my long-held contention that the standard counterpoise correction overcompensates and that we really have no reliable way of correcting for BSSE.

Table 1. Dimerization energies (kcal mol^-1) at MP4(SDTQ)/aug-cc-pVDZ.¹

The relative energies of the 9 configurations are similar, indicating a very flat potential energy surface. The lowest energy structure is BT-2, and the V-shape configuration is the least favorable of the nine geometries examined.

References

(1) Dinadayalane, T. C.; Leszczynski, J., "Geometries and stabilities of various configurations of benzene dimer: details of novel V-shaped structure revealed " Struct. Chem. 2009, 20, 11-20, DOI: 10.1007/s11224-009-9411-6.