Compound     syn    anti Selected experimental and calculated IR frequencies [cm-1] ΔEs-cis/s-trans [kcal/mol] Barrier height Esyn→anti [kcal/mol]   Barrier height Esyn→anti [kcal/mol] HOMA [au] Exp. Calc.* Exp. Calc. Exp. Calc. nN-H nN-H nC=O nC=O nC=C nC=C 1 pyrole   3485 3600     1532b 1540       0.854 2 R1=H, R=H   3460 3565 1659d 1685 1552b 1553 3.84 16.21 10.38 0.923 3   R1=H, R=H   3576   1712   1554 0.911 4 R1=H, R=OH   3465 3570 1671d 1723 1555c 1556 1.11 11.3 (13.03) 11.94 0.921 5   R1=H, R=OH   3587   1754   1561 0.916 6 R1=H, R=OCH3   3465b 3480b 3571 1719d 1705 1554b 1557c 1555 1.06 (1.10) 12.35 (12.43) 11.36 0.917 7   R1=H, R=OCH3   3589 1701d 1736   1559 0.913 8 R1=H, R=NH2   3415 3336c 3559 1644c 1603c 1683 1552c 1550   4.91 10.17 6.60 0.917 9   R1=H, R=NH2 3582   1710   1560 0.882 10 R1=H, R=NHCH3     3571   1665   1558 6.02 8.75 3.33 0.914 11   R1=H, R=NHCH3   3581   1690   1548 0.875 12 R1=H, R=N(CH3)2   3450 3552 1612 1623 1548a 1547b 1544   4.95 7.03 2.07 0.911 13   R1=H, R2=N(CH3)2 3587   1697   1560 0.886 14 R1=H, R=CH2Cl   3458d 3562 1662d 1695 1546a 1545c 1546 5.15( 3.1) 13.9 10.8 0.923 15   R1=H, R=CH2Cl 3594 1683d 1714   1552 0.899 16 R1=H, R=CHCl2   3457d 3561 1658d 1692 1543a 1542c 1546 4.19 (2.4) 13.89 11.45 0.927 17   R1=H, R=CHCl2 3562 1683d 1710   1548 0.921 18 R1=H, R=CCl3   3456d 3563 1670b 1687 1538a 1539b 1538c 1542 2.55 (2.51) 12.85 10,33 0.926 19   R1=H, R=CCl3 3583 1679d 1681b 1708   1545 0.922 20 methylpyrole             1517       0.865 21 R1=CH3, R=H       1670 1689 1528b 1531 4.52 (3.47) 15.31 (15.26) 11.79 0.914 22   R1= CH3, R=H     1694   1533 0.906 23 R1= CH3, R2=OH       1670 1724 1530d 1536b 1531 1.93 11.19 9.25 0.913 24   R1= CH3, R=OH       1734   1536 0.904 25 R1= CH3, R=OCH3       1711 1706 1532b 1531 1.93 10.37 8.44 0.910 26   R1= CH3, R=OCH3     1717   1537 0.902 27 R1= CH3, R=NH2       1639c 1683 1529c 1528 5.54 9.67 7.21 0.908 28   R1= CH3, R=NH2     1695   1537 0.882 29 R1= CH3, R=NHCH3         1667   1537 4.32 8.98 4.65 0.908 30   R1= CH3, R=NHCH3     1677   1542 0.877 31 R1= CH3, R=N(CH3)2       1630d 1635 1537a 1536b 1529 3.85 13.51   0.902 32   R1= CH3, R=N(CH3)2     1660   1542 0.870 33 R1= CH3, R=CH2Cl       1662d 1694 1527a 1522c 1526 6.61 (5.68) 12.74 6.67 0.909 34   R1= CH3, R=CH2Cl     1683d 1693   1527 0.886 35 R1= CH3, R=CHCl2       1658d 1692 1526a 1524c 1526 6.64 (6.97) 13.5 6.59 0.905 36   R1= CH3, R=CHCl2     1683d 1690   1527 0.889 37 R1= CH3, R=CCl3         1690 1524a 1522c 1526 8.08 (8.04) 11.62 4.48 0.899 38   R1= CH3, R=CCl3     1681d 1692   1522 0.878 a CHCl3, b CCl4, c KBr, d cyclohexane

Table 1: Experimental and calculated (B3LYP/6-311++(d,p)) IR stretching frequencies of the C=O, C=C and N-H groups, for syn and anti pyrrol-2-yl carbonyl conformers (1-38). ΔE is the difference between the energies of the syn and anti-form. HOMA [49] is the aromaticity indices.