Table 2.

Comparison of the AT and GC base pair HBs in gas phase and DNA ${}^a$.

Base Pair	X–H Bond		Y⋯H Bond		X⋯Y Distance
	d	k ${}^{\mathit{a}}$	d	k ${}^{\mathit{a}}$	d${}_{\mathit{calc}}$	d${}_{\mathit{exp}}{}^{\mathit{b}}$
	(Å)	(mdyn/Å)	(Å)	(mdyn/Å)	(Å)	(Å)
AT1${}_{DNA}$
N–H⋯O	1.020	6.747	2.108	0.128	3.122	3.050
N⋯H–N	1.050	4.496	1.764	0.383	2.812	2.776
C–H⋯O	1.084	5.772	2.504	0.156	3.334	3.468
AT2${}_{DNA}$
N–H⋯O	1.015	7.014	2.144	0.102	3.143	2.981
N⋯H–N	1.065	3.338	1.638	0.420	2.700	2.761
C–H⋯O	1.085	5.701	2.385	0.141	3.234	3.475
AT3${}_{gas}$
N–H⋯O	1.022	6.554	1.903	0.201	2.921	-
N⋯H–N	1.047	4.673	1.777	0.318	2.824	-
C–H⋯O	1.087	5.635	2.738	0.060	3.570	-
GC1${}_{DNA}$
N–H⋯O	1.020	6.508	1.855	0.267	2.866	2.880
N–H⋯N	1.035	5.508	1.831	0.511	2.851	2.912
O⋯H–N	1.030	5.663	1.748	0.282	2.763	2.852
GC2${}_{DNA}$
N–H⋯O	1.027	6.096	1.758	0.387	2.784	2.789
N–H⋯N	1.038	5.379	1.855	0.517	2.892	2.875
O⋯H–N	1.025	6.241	1.868	0.230	2.888	2.839
GC3${}_{gas}$
N–H⋯O	1.022	6.418	1.866	0.264	2.888	-
N–H⋯N	1.033	5.695	1.891	0.486	2.924	-
O⋯H–N	1.034	5.496	1.750	0.283	2.785	-

${}^a$ QM/MM calculations in DNA: the base pair AT1${}_{DNA}$, AT2${}_{DNA}$, GC1${}_{DNA}$, GC2${}_{DNA}$, the $\omega$B97X-D/6-31++G(d,p)/AMBER level of theory; QM calculations in the gas phase: the base pair AT3${}_{gas}$ and GC3${}_{gas}$, the $\omega$B97X-D/6-31++G(d,p) level of theory. The left atomic symbol of the base pair label corresponds to the purine basis (A and G), and the right atomic symbol corresponds to the pyrimidine basis (T and C); the X and Y symbols correspond to the hydrogen donor and acceptor atoms, respectively. ${}^b$ Taken from the experimental X-ray structure [135].