R=H R=OH R=CH3 R=CH2Cl R=CHCl2 R=CCl3 R=NH2 R1=H R=NH2 R1=CH3 R=NHCH3 R=OH syn anti syn anti syn anti syn anti B3LYP/6-311++G(d,p) N-H 1.024 1.020 (1.019) 1.022 1.022 1.022 1.019   1.026 1.027     1.01   1.009 (0.996) 1.008 (0.99) C=O dim 1.231 1.225 (1.226) 1.235 1.228 1.227 1.222   1.243 1.235 1.241 1.238 1.251   1.238 (1.237) 1.231 (1.230) H…O 1.835 1.88 (1.892) 1.855 1.855 1.847 1.897   1.865 1.872     1.857   (1.697) (1.688) ∠X-H..O X= N, O 164.4 163.7 (163.9) 161.1 163.2 164.2 164.6   178.6 172.7     179.9   179.7 (178.9) 180.0 (178.6) ΔvC=O (vmon-vdim) 17 21   34 28 25   10 25     7.5 15.6 (53) (60) EHB [kcal/mol] -6.9 -5.9 (-5.7) -6.23 -6.45 -6.63 -5.26   -6.78 -6.96 -6.01 -6.57 -6.98 -7.61 -8.0 (-7.7) -8.5 (-8.3) ρBCP [au] 0.030 0.0263 0.0283 0.0278 0.0282 0.0261   0.0303 0.0298     0.0314   0.0483 0.0498 ρRCP [au] 0.0029 0.0030 0.0036 0.0036 0.0036     0.0048 0.0049     0.0058   0.0081 (0.008) 0.0081 (0.008) Reference [58] [39] [40] [40] [40] [40]   [69] [69] [88] [88]     [39] [84]

Table 2: Geometrical, and topological parameters of X–H…O=C hydrogen bonds (in Å and degrees) with H-bond energies EHB in kcal mol-1 of 2-acylpyrrole dimers calculated at the B3LYP/6-311++G(d,p) level of theory; data calculated at B3LYP/6-311+G* are given in parentheses. The ring critical point was created due to double hydrogen bond formation through the C=O and N-H bonds or through C=O...H-X.