Role of Nitrogen Substitution in Phenyl Ring on Excited State Intramolecular
Proton Transfer and Torsional Rotation of Keto Form of
2-(2'-Hydroxyphenyl)benzimidazole: A Theoretical Study
Francis A. S. Chipem and G. Krishnamoorthy*
Department of Chemistry
Indian Institute of Technology Guwahati
Guwahati 781039
India
Supplementary Material
FIGURES
FIG. S1: Frontier molecular orbitals of K-1 and K-2 forms of 2',3'-HPyBI involved in the
electronic transition.
FIG. S2: Water molecule assisted proton transfer in 2',3'-HPyBI.
Planar Keto
Twisted Keto
FIG. S3: Numbering scheme of keto tautomers
HPBI
2',3'-HPyBI
3',4'-HPyBI
4',3'-HPyBI
3',2'-HPyBI
5',4'-HPymBI
FIG. S4: Simulated potential energy surfaces for torsion of tautomers between the two aromatic
rings obtained from TDDFT calculations.
TABLES
TABLE S1: Dipole moments and selected structural parameters of different enols and keto
form of molecules in S0 and S1 states.a
Parameter
cis -Enol
Open-enol
trans-Enol
Keto
HPBI
3.91 (3.77)
3.44 (2.17)
4.69 (3.93)
5.92 (4.92)

ROH (17,18)
0.998 (0.964)
0.971 (0.948)
0.970 (0.948)
1.630 (1.963)
RNH (1,18)
1.732 (1.825)
1.057 (1.001)
ROH (17,10)
2.038 (2.097)
RNO (1/3,17)
2.626 (2.671)
2.729 (2.734)
2.698 (2.711)
2.519 (2.651)
2',3'-HPyBI
4.15 (3.73)
3.64 (6.21)
2.26 (2.58)
8.51 (7.84)

ROH (17,18)
0.998 (0.964)
0.976 (0.952)
0.976 (0.952)
1.655 (1.963)
RNH (1,18)
1.735 (1.826)
1.052 (1.001)
ROH (17,10)
2.069 (2.126)
RNO (1/3,17)
2.633 (2.677)
2.766 (2.761)
2.727 (2.731)
2.529 (2.639)
3',4'-HPyBI
2.04 (3.59)
5.49 (5.87)
3.81 (5.37)
8.39 (7.17)

ROH (17,18)
0.996 (0.965)
0.972 (0.9488)
0.971 (0.949)
1.662 (1.983)
RNH (1,18)
1.755 (1.848)
1.053 (1.001)
ROH (17,10)
2.064 (2.111)
RNO (1/3,17)
2.642 (2.686)
2.760 (2.771)
2.722 (2.731)
2.538 (2.673)
4',3'-HPyBI
1.26 (0.88)
3.10 (4.04)
5.95 (5.86)
6.80 (5.31)

ROH (17,18)
1.001 (0.962)
0.972 (0.948)
0.970 (0.948)
1.667 (2.007)
RNH (1,18)
1.732 (1.868)
1.053 (1.000)
ROH (17,10)
2.070 (2.137)
RNO (1/3,17)
2.629 (2.704)
2.757 (2.770)
2.722 (2.744)
2.543 (2.681)
3',2'-HPyBI
3.98
(1.36)
2.27
(2.23)
6.55 (5.77)
4.57 (4.02)

ROH (17,18)
0.997 (0.963)
0.971 (0.948)
0.970 (0.948)
1.767 (2.057)
RNH (1,18)
1.784 (1.881)
1.040 (0.9997)
ROH (17,10)
2.021 (2.065)
RNO (1/3,17)
2.674 (2.721)
2.830 (2.825)
2.687 (2.689)
2.594 (2.704)
5',4'-HPymBI
0.63 (2.69)
3.64 (5.12)
5.65 (6.89)
7.27 (6.55)

ROH (17,18)
0.996 (0.963)
0.972 (0.949)
0.971 (0.949)
1.791 (2.083)
RNH (1,18)
1.807 (1.927)
1.039 (1.000)
ROH (17,10)
2.044 (2.082)
RNO (1/3,17)
2.690 (2.752)
2.865 (2.876)
2.710 (2.7143)
2.613 (2.733)
a
Bond lengths and angles are in Å and degree respectively. Dipole moment in Debye. Values in parentheses are that
of S1 state.
TABLE S2: Dipole moments and selected structural parameters of 2',3'-HPyBI for the keto
forms K-1 and K-2 in S0 and S1 states (within parentheses).
Parameter

ROH (17,18)
ROH (17,10)
RNO (1/3,17)
K-1
6.72 (5.71)
2.389 (2.353)
2.860 (2.858)
K-2
4.83 (2.09)
2.413 (2.389)
1.991 (2.100)
2.711 (2.737)
TABLE S3: Energy difference and barrier for proton transfer reaction (eV) in cyclohexane and
acetonitrile.a
Solvent
ΔEb
Energy Barrier
cis-Enol
Keto
HPBI
0.7454 (-0.1683) 0.8816 (0.3921)
0.1363 (0.5604)
0.4980 (-0.2347) 0.7358 (0.3784)
0.2378 (0.6131)
2',3'-HPyBI
0.1416 (0.5708)
Cyclohexane 0.7192 (-0.0941) 0.8607 (0.4767)
0.2546 (0.6142)
Acetonitrile 0.4076 (-0.1826) 0.6622 (0.4316)
3',4'-HPyBI
0.1581 (0.5784)
Cyclohexane 0.7924 (-0.1461) 0.9505 (0.4323)
0.2651 (0.6561)
Acetonitrile 0.5136 (-0.2304) 0.7787 (0.4257)
4',3'-HPyBI
0.1694 (0.6598)
Cyclohexane 0.6437 (-0.1574) 0.8131 (0.5024)
0.2731 (0.6344)
Acetonitrile 0.3848 (-0.1439) 0.6579 (0.4905)
3',2'-HPyBI
0.2463 (0.6706)
Cyclohexane 0.7285 (-0.0729) 0.9748 (0.5977)
0.3132 (0.6971)
Acetonitrile 0.4995 (-0.1338) 0.8126 (0.5633)
5',4'-HPymBI
0.2564 (0.7780)
Cyclohexane 0.7948 (-0.1155) 1.0513 (0.6624)
0.3506 (0.7752)
Acetonitrile 0.5342 (-0.1476) 0.8848 (0.6276)
a
Values in parentheses are that of S1 state. bEnergy difference between cis-enol and keto-tautomer.
Cyclohexane
Acetonitrile
TABLE S4: Selected bond lengths and angles of the tautomers in the optimized S0 and S1
states (for the atom numbering, refer FIG. S3).
Geometry
Parameters
Keto
S0 State
S1 State
Twisted Tautomer
S0 State
S1 State
N1 C2
N3 C2
C2 C11
C11 C12
C11 C16
C12 O17
N1 C2 C11 C12
C5 N1 C2 C11
1.35148
1.36952
1.41744
1.47001
1.41041
1.25922
-0.01990
179.99811
2',3'-HPyBI
1.35685
1.36830
1.40534
1.51001
1.40079
1.22002
-0.01137
179.99047
1.35087
1.35089
1.44595
1.47101
1.39493
1.24600
87.65675
-174.09121
1.40842
1.41004
1.43209
1.49396
1.42450
1.19754
66.48273
-152.49942
N1 C2
N3 C2
C2 C11
C11 C12
C11 C16
C12 O17
N1 C2 C11 C12
C5 N1 C2 C11
1.35180
1.36768
1.41812
1.45214
1.41788
1.26986
0.00103
-180.00000
3',4'-HPyBI
1.35553
1.36650
1.40491
1.48856
1.39092
1.23010
-0.00060
-179.99689
1.34938
1.34939
1.44780
1.45497
1.40709
1.25400
87.47633
-174.45755
1.40602
1.40959
1.43868
1.47364
1.41547
1.20461
67.61629
-150.05776
N1 C2
N3 C2
C2 C11
C11 C12
C11 C16
C12 O17
N1 C2 C11 C12
C5 N1 C2 C11
1.35067
1.36710
1.41879
1.46159
1.41327
1.26705
0.00302
-179.99962
4',3'-HPyBI
1.36125
1.37070
1.40076
1.49712
1.40157
1.22591
-0.00121
-179.99877
1.35019
1.35018
1.44495
1.46559
1.40196
1.25021
88.08348
-175.52207
1.40832
1.41025
1.42684
1.48964
1.44774
1.20418
68.30817
-153.17114
N1 C2
N3 C2
C2 C11
C11 C12
C11 C16
C12 O17
N1 C2 C11 C12
C5 N1 C2 C11
1.35058
1.35872
1.41819
1.45276
1.35946
1.26741
0.00077
-180.00000
3',2'-HPyBI
1.35406
1.35709
1.40972
1.49196
1.33170
1.22956
0.00283
179.99906
1.34614
1.34615
1.46840
1.44735
1.34348
1.25833
89.70911
-178.23576
1.40567
1.40948
1.43429
1.48415
1.35814
1.20610
68.10541
-151.66542
N1 C2
N3 C2
C2 C11
C11 C12
C11 C16
C12 O17
N1 C2 C11 C12
C5 N1 C2 C11
1.34957
1.35660
1.41985
1.44233
1.35941
1.26493
-0.00564
179.99927
5',4'-HPymBI
1.35250
1.35523
1.40794
1.47674
1.33215
1.22972
0.00647
179.99212
1.34526
1.34525
1.46778
1.44199
1.34680
1.25397
89.04696
-177.50183
1.40265
1.40880
1.43584
1.47002
1.35666
1.20366
68.09616
-149.74740
TABLE S5: Solvent effects on the excitation (eV) of twisted keto
Molecule
HPBI
2',3'-HPyBI
3',4'-HPyBI
4',3'-HPyBI
3',2'-HPyBI
5',4'-HPymBI
Cyclohexane
1.0790
1.3286
1.2111
1.6878
1.2726
1.4522
Acetonitrile
1.6294
1.8919
1.8263
2.2644
1.7941
2.0329