Table 2.

Main conformers^a of 12a and 27a, calculated by B3LYP/6-311+G(d,p) with PCM in water: energies and geometrical features of the hydrogen bonds.

Conformer	Energy (kcal/mol)	Carbon chainb	Hydrogen bonds Atoms involved	dH-Acc (Å)	θDon-H-Acc (°)
12a
1	0.00	equat.	H(POax) -Oax(P′)	1.82	154
			H(POeq)-N	1.68	157
2	0.59	axial	H(POax) -Oax(P′)	1.89	154
			H(POeq)-N	1.73	155
3	6.50	equat.	H(POax) -Oax(P′)	1.86	154
			H(N)-Oeq(P)	2.25	139
4	8.49	equat.	H(N)-Oeq(P)	2.19	139
27a
1	0.00	equat.	H(POax) -Oax(P′)	1.62	161
			H(Oax)-N	2.01	124
			H(N)-Oeq(P)	2.24	134
2	1.77	equat.	H(Oax)-Oeq(P)	1.91	135
			H(POeq)-N	1.78	153
3	2.17	axial	H(Oeq) -Oax(P)	1.95	133
			H(POeq)-N	1.67	157
4	3.75	equat.	H(POax) -Oax(P′)	1.62	162
			H(Oax)-Oeq(P)	2.82	108
			H(N)-Oeq(P)	2.14	142
5	4.86	axial	H(POax) -Oax(P′)	1.59	160
			H(Oeq)-Oeq(P)	2.25	127

^aFor each of the nine H-bond/carbon chain arrangements, the conformer with the orientation of the N-methyl group with lower energy is shown.

^bThe designation of substituents as axial or equatorial was made considering the presence of a six-membered cycle formed by the atoms C-1–P-1–O–Mg–O–P-2.