Intramolecular OâHÂ·Â·Â·O C Hydrogen Bond Energy via the Molecular

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Intramolecular O-H···O=C Hydrogen Bond Energy via the Molecular Tailoring Approach to RAHB Structures Danuta Rusinska-Roszak J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b02343 • Publication Date (Web): 18 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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The Journal of Physical Chemistry

Intramolecular O-H···O=C Hydrogen Bond Energy via the Molecular Tailoring Approach to RAHB Structures

Danuta Rusinska-Roszak Institute of Chemical Technology and Engineering, Poznan University of Technology, ul. Berdychowo 4, 60-965 Poznan, Poland

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ABSTRACT

A method for the calculation of the intramolecular hydrogen bond (HB) energy (EHB) by molecular tailoring approach for hydroxycarbonyl aliphatic compounds has been used for compounds with resonance-assisted hydrogen bonding (RAHB). The intramolecular hydrogen bond energies estimated for 229 structures (of 186 compounds) range from 10.4 to 26.3 kcal/mol and show correlation with the geometry descriptors of hydrogen bonds, with the calculated frequencies as well as with topological parameters obtained from the atoms in molecules (AIM) theory. These correlations differ significantly from obtained formerly for saturated nonenolizable structures and prove the special character of the resonance-assisted hydrogen-bonded systems.

1. INTRODUCTION Hydrogen bonding is well recognized as one of the major noncovalent forces which play a prominent role in supermolecular and template chemistry and is a crucial issue in the study of biologically important molecules.1-3 The predictable formation of certain intramolecular motifs has a significant influence on the ability of the molecule to engage in intermolecular hydrogen bonding,4 and affects molecular properties including biological and pharmacological activities. Structural behavior of biochemical systems and nature of hydrogen-bonded liquids can be better understood when the concept of single hydrogen bonds is adopted and their detection and quantification is possible. The energy of intermolecular interactions is simply measured as a difference between the energies of a H-bonded structure and its components. This approach cannot be used to study intramolecular interactions, and despite the fact, that the hydrogen bonds are qualitatively well understood,3-7 it is generally admitted that a simple method of calculation and quantitative data are needed.8 Numerous experimental9,10 and theoretical11-13 studies have been devoted to explore intramolecular HB of the cis-enol tautomer of malonaldehyde14 as a representative member of a special class of compounds containing a resonance-assisted hydrogen bond (RAHB).3 RAHB is typically classified as a π-conjugated ring motive for which characteristic changes in the geometrical or electronic properties are observed, for example, the elongation of formally double bonds and the shortening of formally single bonds. The model first proposed by ACS Paragon Plus Environment

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Gilli15,16 suggest that π–electron delocalization between the donor and acceptor atoms is responsible for enhancing the intramolecular HB strength in malonaldehyde, β-diketones and derivatives.14 Many authors7,17,18 have established that RAHB systems fulfill the conditions of the aromatic π-electron delocalization by its aromaticity numerical descriptors (aromaticity indices)7 therefore, they can be treated as quasiaromatic.19 Direct and indirect attempts for determining the strength of the intramolecular hydrogen bond have been reported in the literature.11,15,16,20-22 Usual indicators of the strength of the hydrogen bond are: the shortening of the distance d(O···O) and the lengthening of the covalent bond d(O-H), the decrease of the stretching ν(OH) frequency and the downfield shift of the δH NMR peaks,23,24 the increase of Lewis acidity of the proton donor, and the growth of Lewis basicity of the proton acceptor.2 Although all these factors change regularly with the strength of hydrogen bond, generally there is no possibility to evaluate its numerical value with one equation. In the special case of resonance-assisted hydrogen bonds, the parameters describing the π-electron delocalization within the ring due to intramolecular H-bond formation may be applied because they often correlate with the Hbond energy.15,16,25,26 The electron densities ρ at the selected bond critical points (BCP) derived from the Bader27 theory are especially useful as descriptors of H-bonds. The theory of atoms in molecules (AIM),28,29 along with ab initio DFT or MP230-32 calculations with the analysis of the geometrical, energetic and topological data, describe the strength of H bridges by different types of factors.12,26,33-38,43,44 Especially the ring critical points (RCP) can be useful for the comparing of the HB energy in compounds with RAHB36,37 even in two-ring (chelated intramolecular HB) systems. A large amount of theoretical studies were taken to describe the geometry and electronic nature of the RAHB investigated systems, from semiempirical methods,15,45 by standard B3LYP density functional theory (DFT) calculations9,17,18,23,24 with basis sets including the polarization and diffuse functions,23,41 to the high-level ab initio calculations performed at the MP2/6-311++G(d,p) level of theory.7,39 The energy of such optimized structures stabilized by the intramolecular hydrogen bond was used to estimate its strength by different comparative methods: cis-trans analysis,12,13,18 isodesmic reactions46 and conformational analysis with gradual or total rotation of the H-bond donor or acceptor.11,22,23 The first of these methods is ACS Paragon Plus Environment

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simplistic, because the energetic stabilization of the H-bonded conformer includes several contributions, such as the balance between attractive and repulsive terms, steric constraints of bulky groups, conjugation and other interactions, and this situation changes radically after turning out of the hydroxyl group. In the isodesmic method the subtle differences in the electron density of O-H···O=C bonded molecules may not be noticeable. Consequently, the calculation of the energy of intramolecular hydrogen bonds EHB in malonal and its derivatives structures show a significant disparity of results for different methods from 2.77 to 43.63 kcal/mol47 and from 4.9 to 18.0.14 The other procedures of the EHB calculations recently described44 may be applied only for some groups of compounds. Therefore different authors13,26,37 calculating the energy using these methods called them as binding energy, energy increase upon H-bond removal or the energy difference between the closed and opened structures of the molecule instead of the strength of the H-bond. In our previous article8 the new method of calculation of the intramolecular hydrogen bonding was proposed, as the enhancement of the molecular tailoring method (MTA) introduced by Gadre for fragmentation of supermolecules,48 and applied by Deshmukh et al.49,50 to the calculation of EHB in polyalcohols, with error ~0.5 kcal/mol. Our8 modification of method extends the area of its employing for many hydroxycarbonyl compounds, from saturated, cyclic and acyclic, to RAHB systems. It was checked by comparing the results with effects of applying it for some intermolecular HBs and by using it in the different 6-, 7- and larger membered hydrogen bonds in over 150 structures, omitting in the first part the RAHB, phenolic and five-membered systems. The distinction was made because of specific relationships of the intramolecular hydrogen bond with some descriptors of HB strength, as changes of its geometrical parameters, and of some topological factors, especially associated with the electron density in examined structures. To our best knowledge, MTA method referred here and earlier8 was never used before for the calculation of the hydrogen bond energy, as the procedure universal for different types of intramolecular H bonding.

2. METHODOLOGY In this study, the geometries of 229 structures of β-hydroxy- α,β-unsaturated aliphatic ketones, aldehydes,


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acids, halides, amides and esters with intramolecular hydrogen bond(s) forming the six-membered rings were built and optimized as previously at the DFT B3LYP/6-311++G(d,p) level of theory using the Gaussian 09 program package.51 All structures were checked by the vibrational analysis at this level and were found to represent true energy minima. The calculated νOH and νC=O frequencies were identified with the GaussView 5.0.9 package, without correction for the zero-point energies and without scaling the force constants to produce experimental vibrational frequencies. At the same level, the absolute proton shielding for each structure and TMS (standard tetramethylsilane) were obtained using the gauge-including-atomic-orbital (GIAO) method.52 The SCF GIAO magnetic isotropic shielding tensors of H-bonded hydrogen atoms were used with the consequent calculation of the chemical shifts δH [ppm] by subtraction of the averaged shielding of the TMS hydrogen atoms. Afterwards, the electron correlation was included via the Møller-Plesset treatment of the second order (MP2) in which each structure was finally optimized at MP2(FC)/6-311++G(2d,2p), and its energy was specified at MP2(full)/6-311++G(2d,2p) calculations. The topological properties of the electron density at the bond critical points (BCPs) were characterized using the atoms in molecules methodology (AIM) with the AIM200028 and AIMAll29 program packages for every fully optimized compound. As in previous paper, to describe the nature of the hydrogen bonding we chose the electron density ρBCP at the (3,-1) bond critical point, its Laplacian ∇2ρBCP for the BCP between the donor OH hydrogen atom and the O=C oxygen atom of the acceptor, and the electron density ρRCP at the ring critical point in the center of the six membered ring of the HB. The geometry of each intramolecular hydrogen bond was provided by the length of the O-H covalent bond (dOH), the length of the HB as H···O (rHB), the angle of the HB as O-H···O (φHB) and the distance between both of the oxygen atoms as O···O (dO···O). This information, together with calculated frequency of O-H and C=O stretching and the hydrogen chemical shift is collected in Table S2, along with the AIM characteristics of the analyzed structures.


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For the estimation of the O-H···O=C intramolecular hydrogen bond energy, a systematic fragmentation of each optimized molecule was performed using the same methodology as previously.8 Following Deshmukh,50 the O-H group is replaced by the hydrogen atom at appropriate direction and with a constraint distance of 1.1 Å. The C=O group is removed with the part of the molecule “behind” Cα, at the site opposite to the OH group. It is very important that the hydrogen atom could replace only the sp3-hybridized atoms of oxygen or carbon and that after this operation any hydrogen atoms remain closer than 2.2 Å (double the van der Waals radius). The energies of these constructed parts (Scheme 1) were calculated without optimization at the MP2(full)/6311++G(2d,2p) level and were used to calculate the strengths of the hydrogen bonds, according to the given equation. EHB = EM1 + EM2 – EM3 – EMH Scheme 1. MTA Fragmentation Scheme for Enolic Intramolecular HB Energy Calculation


6

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EHB = EM1 + EM2 – EM3 - EMH

To ensure the validity of the tailoring scheme, in some testing calculations the fluorine atom, which mimics lone electron pair character on oxygen atom and provides n-π conjugation in structures M1 and M3, was used as the cap instead of hydrogen atom. We checked whether the elimination of the oxygen p-electrons during tailoring is not disruptive for whole RAHB system. The energies of HB were estimated for a few selected Hbonded structures (Table S3 in Supporting Information), for bonded 2-hydroxypropenal and for seven its nonbonded conformers (Table S4 in Supporting Information). One can notice that for most of structures the difference between F and H substitution results is not essential. Moreover the test of non-bonded isomers of 2hydroxypropenal shows that in structures of (Z) configuration the breaking of (Z) conjugation of O=C-C=C-OH system does not affect the accuracy of the calculations. In the present study, the geometries of several aliphatic β-hydroxy- α,β-unsaturated ketones, aldehydes, acids, halogenides, amides and esters with the intramolecular resonance-assisted hydrogen bonds were investigated. 3. RESULTS The one hundred eighty six compounds analyzed herein are the enolic derivatives of basic structures of malonaldehyde 1 or pentane-2,4-dione 5 with EHB values of 14.5 and 15.2 kcal/mol, respectively. The majority of the derivatives considered in this study has an energy in the range of 14-16 kcal/mol. The average value of the energy of the intramolecular hydrogen bonds calculated by the MTA is 15.2 kcal/mol and should be classified as strong.5 The energies of more than two hundred resonance-assisted hydrogen bonds in aliphatic systems as single and chelated hydrogen bonds are shown in Tables 1, 2 and 3. Table S1 of the Supporting Information presents the systematic (and occasionally trivial) names of the examined compounds and the energies of the intramolecular HB for these compounds, calculated by us by MTA method. The energies of hydrogen bond formation are compared as positive values. The list of compounds, geometric parameters, spectral and selected AIM properties of each examined structure are collected in Table S2. ACS Paragon Plus Environment

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3.1. β-Ketoenols Table 1 contain the essential structural parameters of stable conformers of resonance-assisted hydrogenbonded aldehydes, ketones, acids, esters, amides and halogenides, and the energies EHB calculated herein are compared (when possible) with the values described in the literature. The compounds which meet the three criteria were classified to this RAHB group: the proton donor is the enolic OH group (although not only β-ketoenolic), the HB electron system satisfies the Hückle’s rule and the HB ring is completely flat. Example compound which do not meet these conditions is non-planar enol (Z)-3tert-butyl-4-hydroxypent-3-en-2-one with EHB = 13.7 kcal/mol, which was placed between saturated compounds (ref 8, compound 43). Table 1. MTA Energy of Intramolecular Hydrogen Bond (EHB in kcal/mol) for β-Hydroxy α,β-Unsaturated Carbonyl Compounds

no

R1a

R2

R3a,b

E relc ─EHB

─EHB lit

1

H

H

H

―

13.563, 10.9010, 14.0011, 15.213, 11.915, 12.9618, 16.9223, 13.326, 12.1537, 12.9440, 5.05, 14.81, 14.5 17.1647, 14.0147,55, 13.153, 12.1454

2

H

H

Me

―

15.1 14.726, 13.7754

3

Me

H

H

―

14.6 14.126, 13.1354

4

H

Me

H

―

14.0 14.026, 15.4655

5

Me

H

Me

―

16.2311, 18.0813, 12.7615, 15.021, 15.626, 15.8633, 14.7854, 15.1355, 15.2 12,556, 15.8757

6

Me

Me

H

―

15.2


8

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7

H

Me

Me

―

15.1

8

Me

Me

Me

―

16.2 19.0755

9a

iPr (H c)

H

iPr (H c)

0.0

16.0

9b

iPr (H g)

H

iPr (H c)

0.4

15.4

10

iPr

Me

iPr

―

16.5

11

Me

H

iPr

―

15.5

12

iPr

H

Me

―

15.6

13a

tBud

H

tBud

0.0

15.3 19.7215, 15.621, 17.9058

13b

tBue

H

tBue

0.4

17.3

14

Me

H

tBu

―

15.2 20.159

15

tBu

H

Me

―

17.2 15.521, 16.5760

16

H

tBu

H

―

16.4 14.8011, 15.4955

17

Me

iPr

Me

―

17.0

18

H

tBu

Me

―

16.4

19

Me

tBu

H

―

15.8

20

Ph

H

H

―

15.2 14.653

21

H

Ph

H

―

14.5 15.4511

22

H

H

Ph

―

15.3 17.253

23

Ph

H

Ph

―

19.843, 16.421, 17.0025, 19.153, 16.0 16.1557

24

Ph

Me

Ph

―

15.8

25

Ph

H

Me

―

15.9 16.234, 16.138, 16.0761

26

Me

H

Ph

―

15.3 16.221, 16.334, 16.0761

27

Me

Ph

Me

―

15.1 16.2211

28

Me

4-OMeC6H4

Me

―

15.1

29a

H

-N=N-Ph strans

H

0.0

14.0

29b

H

-N=N-Ph scis

H

1.3

15.4

30

Me

―

15.0

-CH2-CH2-CH2-CH2-


18.562 18.3562

9


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H

31 32

-CH2-CH2-CH2-CH2-

-CH2-CH2-CH2-CH2-

H

0.0

14.1

1.3

16.2

33

Me

-CH2-CH2-CH2-

―

15.8

34

H

-CH2-CH2-CH2-

1.3

15.3

0.0

13.8

3.0

11.1

H

0.0

8.2

-CH2-CH2-CH2-

35 36

H

-CH2-CH2-CH2-CH2-

37

H

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38

F

H

H

―

11.1 9.426, 9.1437

39

H

H

F

―

23.4 21.026, 13.4737

40

H

F

H

―

13.7 9.8110, 11.026, 9.7337, 13.7255

41

Me

F

Me

―

14.4 14.8655

42

Cl

H

H

―

11.3 9.426, 9.2437

43

H

H

Cl

―

19.5 19.626, 12.4937

44

H

Cl

H

―

13.9 12.726, 10.8337

45

Me

Cl

Me

―

15.9 16.4333, 16.4263

46

Br

H

H

―

11.4

47

H

H

Br

―

18.9

48

H

Br

H

―

13.7

49

Me

Br

Me

―

16.1

50

F

H

F

―

14.5 13.0955

51

F

F

H

―

11.4

52

H

F

F

―

20.0 13.2310

53

Cl

H

Cl

―

13.8

54

Cl

Cl

H

―

12.6

55

H

Cl

Cl

―

16.6 14.0410

56

Br

H

Br

―

13.5

57

Br

Br

H

―

12.6

58

H

Br

Br

―

16.3 14.7310

59

CH=CH2 s-cis

H

Me

―

16.3


10

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60

CF3

H

Me

―

15.2955, 15.959, 13.1364, 13.665, 14.1 13.766

61

Me

H

CF3

―

14.7 15.2555, 13.5864, 13.165, 13.566

62

CF3

H

tBu

―

14.3 17.159, 13.965, 15.8367

63

tBu

H

CF3

―

14.8 15.4767

64

CF2Cl

H

Me

―

14.4 13.866

65

Me

H

CF2Cl

―

15.0 13.566

66

CF3

H

Ph

―

14.3 16.265

67

Ph

H

CF3

―

15.2 13.965

68

2-C4H3S

H

CF3

―

14.6

69

CF3

H

2-C4H3S

―

15.8

70

2-C4H3O

H

CF3

―

14.3

71

CF3

H

2-C4H3O

―

16.0 13.568

72

CF3

H

CF3

―

13.4 13.5555, 10.1564

73

NH2

H

H

―

13.3 16.569

74

H

H

NH2

―

20.3 21.269

75

NH2

H

Me

―

13.8

76

Me

H

NH2

―

23.8

77

NH2

NO2

H

―

18.8

78

NH2

NO2

NH2

―

37.3923, 27.938,13.05, 33.08, 30.25, 43.6347, 26.769, 27.070, 21.2 27.871

79a

H

H

OH trans

0.0

26.3

79b

H

H

OH cis

2.4

21.0 3.72, 11.45,11.60, 13.4247

80

Me

H

OMe cis

―

21.3

81

Ph

H

OEt cis

―

20.3 16.4972

82a

Me

H

OCH2Ph cis ┴

0.0

21.2 17.2371

82b

Me

H

OCH2Ph cis ║

1.6

21.0 16.6171

83a

OH trans

H

H

0.0

11.7 13.5943

83b

OH cis

H

H

4.8

13.0 13.593


11


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84a

OH trans

H

OH trans

0.0

16.1

84b

OH cis

H

OH trans

4.4

18.2

84c

OH trans

H

OH cis

2.0

15.2

84d

OH cis

H

OH cis

7.1

17.0

85a

OMe cis

H

OH cis

0.0

14.8

85b

OMe cis

H

OH trans

3.3

20.9

86a

OH trans

H

OMe trans

0.0

17.6

86b

OH trans

H

OMe cis

0.4

14.1

87a

OMe trans

H

OH trans

0.0

15.8

87b

OH cis

H

OMe trans

4.9

19.9

88a

OMe trans

H

OMe trans

0.0

17.1

88b

OMe cis

H

OMe trans

4.9

23.3

88c

OMe trans

H

OMe cis

0.2

13.8

88d

OMe cis

H

OMe cis

6.2

17.5

89a

COOMe strans, trans

H

OMe cis

0.0

19.2 14.0822

89b

COOMe s-cis, trans

H

OMe cis

1.5

19.8 14.6722

89c

COOMe strans, cis

H

OMe cis

6.6

18.1 13.0922

90a

OMe trans

H

Me

0.0

11.7

90b

OMe cis

H

Me

6.4

15.0

91a

OEt trans

H

Me

0.0

11.8

91b

OEt cis

H

Me

5.6

15.0

92a

OCH2Ph trans ┴

H

Me

0.0

11.8 14.8473

92b

OCH2Ph trans ║

H

Me

1.0

11.8 15.0073

92c

OCH2Ph cis ┴

H

Me

5.1

14.5 16.7373

92d

OCH2Ph cis ║

H

Me

7.3

15.1 17.1173

93a

OEt trans

H

Ph

0.0

11.9 14.82, 13.6472


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93b


OEt cis

H

Ph

5.8

15.2 16.9772

0.0

11.9 9.1122

94a

OMe trans

H

COOMe s-cis, trans

94b

OMe trans

H

COOMe strans, trans

0.7

11.6 5.6922

H

COOMe s-cis, trans

6.0

15.1 11.3022

7.1

14.8 7.9122

94c

OMe cis

94d

OMe cis

H

COOMe strans, trans

94e

OMe trans

H

COOMe s-cis, cis

8.2

11.9 16.8022

H

COOMe s-cis, cis 14.2

15.4 16.7322

94f

OMe cis

95a

OMe trans

-CH2-CH2-CH2-

0.0

12.6

95b

OMe cis

-CH2-CH2-CH2-

10.1

15.4

96a

OMe trans

-CH2-CH2-CH2-CH2-

0.0

11.7

96b

OMe cis

-CH2-CH2-CH2-CH2-

11.4

12.8

97f

OH cis

-C10H12O2-

―

14.9

98g

OH cis

-C11H6O-

―

16.1

99h

OH cis

-C13H8O-

―

15.9

100i

OH trans

―

11.6 5.19, 15.01, 15.90, 17.3847

101a

OH cis

-2-C6H4-CO-

0.0

15.1

101b

OH trans

-2-C6H4-CO

3.2

13.8

Me

―

15.2

102

C2 = N

H

-O-C(CH3)2-O-CO-

103

H

CHO

OH s-cis

―

17.7

104

CH3

COMe s-cis

OH s-cis

―

19.5

105

C3H6-CO-

OH s-cis

―

19.0

106

C2H4-CO-

OH s-cis

―

16.0

107

C2H2-CO-

OH s-cis

―

10.3

108a

-C6H4-2-CO-

OH cis

0.0

11.6

108b

-C6H4-2-CO-

OH trans

7.0

16.6


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-C6H4-2-CO-

109

Me

―

11.8

110a

OH cis

CHO s-trans

H

0.0

16.9

110b

OH trans

CHO s-trans

H

6.8

13.2

111a

H

CHO s-trans

H

0.0

14.5 14.1511, 11.8141, 14.7755

Page 14 of 43

111b

H

CHO s-cis

H

1.0

13.2611, 12.3141, 6.03, 17.34, 16.3 15.83, 19.7047, 15.1155

112a

H

COMe s-trans

H

0.0

14.6

112b

H

COMe s-cis

H

0.7

16.4

113a

Me

CHO s-cis

Me

0.0

18.0 17.9711,18.8155

113b

Me

CHO s-trans

Me

0.1

17.8 19.9711, 16.6655

114a

Me

COMe s-trans

Me

0.0

16.3

114b

Me

COMe s-cis

Me

0.1

17.1

115a

Me

COOMe s-cis

Me

0.0

17.5

115b

Me

COOMe s-trans

Me

0.1

17.4

116a

Me

COOEt s-cis

Me

0.0

17.3

116b

Me

COOEt-trans

Me

0.1

17.4

117aj

H

C3H3O2

H

0.0

14.5 13.7011

117bk

H

C3H3O2

H

0.3

14.3 13.7511

118al

Me

C3H3O2

Me

―

15.2

118bl

H

C5H7O2

H

―

14.3

119m

Me

C5H7O2

Me

―

15.1 18.6511, 17.374

120

H

OH

H

―

14.2

121

Me

OH

Me

―

14.8

122

Me

OMe

Me

―

14.7

123

H

NH2

H

―

14.6 14.369

124

Me

NH2

Me

―

15.1

125

Me

SMe

Me

126

Me

SPh

Me

―

15.3 17.3676

127

H

NO2

H

―

15.5 15.814, 27.2823,27.523

15.2 15.8875


14

Page 15 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


128

Me

NO2

Me

―

16.7 17.6977

129

H

CN

H

―

14.2 12.6, 12.5, 17.4, 8.7, 15.414

130

Me

CN

Me

―

8.02, 21.51, 18.99, 15.4 25.1847,16.2678

131a

tBue

CN

tBue

0.0

18.6

131b

tBud

CN

tBud

2.4

16.4

132

Me

-CO-O-C(CH3)=CH-

―

15.0

133

Me

-CO-CH(OH)-(CH2)2-

―

16.1

134

Me

-CO-(CH2)2-CH(OH)-

―

16.3

135

H

H

CHO s-cis

―

14.4

136

Me

H

COMe s-cis

―

15.0

137

Me

H

CN

―

14.1

138a

H

OH cis

CHO s-cis

0.0

10.6 10.5939

138b

H

OH trans

CHO s-cis

11.1

12.4

139a

Me

OH cis

COMe s-cis

0.0

11.6

139b

Me

OH trans

COMe s-cis

14.1

13.3

140

Ph

OH cis

COPh s-cis

―

12.0

141

NO2

NO2

NO2

―

12.9 5.62, 16.16, 16.35, 18.1847

H

―

15.515, 5.45, 15.66, 16.24, 12.6 18.1747, 10.1579

Me

―

10.4

0.0

13.8 14.2313

1.54

15.7 14.7413

0.0

15.6 16.3513

1.31

16.3 17.3913

0.0

13.7

H

1.9

17.4

142n

H

-CH=CH-CO-

143 144

H

-CO-CH2-CH2-CH2-CH2-CO-

145 146

C2=N

Me

H -CO-CH2-CH2-

-CO-CH2-CH2-

147 148

H

149

-CH2-CH=CH-CH2-

Me

-CH2-CH=CH-CH2-

150

CH=CH2

H

CH=CH2 trans

―

15.2

151

CH=CHPh

H

CH=CHPh

―

15.1


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

152

Me

H

CH=CH2 strans

153o

Me

H

C8H7O2

―

14.2

154p

C9H9O2

H

C9H9O2

―

15.3

155q

Me

-C6H8O2-

―

16.6

156

Me

-C6H8O2-

―

17.0

157

4-NO2C6H4

H

Ph

―

16.0 17.0925

158

4-MeOC6H4

H

Ph

―

15.8 17.3025

159

2,4,6triMeC6H2

H

Ph

―

16.0 17.0925

160

2,4,6triMeC6H2

H

4-NO2C6H4

―

15.7 16.7025

―

16.1 16.7025

―

14.2

161

3-NO2C6H4

H

2,4,6triMeC6H2

162

2-NO2C6H4

H

2,4,6triMeC6H2

―

16.0 15.6025

163

4-OMeC6H4

H

3-NO2C6H4

―

15.6 17.3825

164

3-NO2Ph

H

4-MeOC6H4

―

16.0 17.3825

165

2,4,6triMeC6H2

H

4-MeOC6H4

―

16.0 17.0025

166

3-BrC6H4

H

3-BrC6H4

―

15.8

167

2-HOC6H4

H

Ph

―

14.9

168

4-OMeC6H4

H

4-tBuC6H4

―

15.9

169

4-tBuC6H4

H

4-OMeC6H4

―

18.0

170r

C9H6O6

―

18.3

171s

C12H12O6

―

21.9

a

Page 16 of 43

Conformation cis and trans defined by H-O-C1-C2 or C-O-C1-C2 torsional angle; s-cis and s-trans defined by O=C-C=O torsional angle; c (cis),g(gauche) defines torsional angles of H in isopropyl group H-C-C1-C2 or H-C-C3-O. b║ means substituent parallel to the plane of HB ring, ┴ means substituent perpendicular to the HB ring. cRelative to the most stable conformer (kcal/mol). dR1 and R3 asymmetric. eR1 and R3 symmetric. fCitrinin, cf. ref 8, compd 34. g(Z)-3-hydroxy-1-oxo-1H-phenalene-2-carboxylic acid, cf. ref 8, compd 35. h(Z)-7hydroxy-5-oxo-5H-dibenzo[a,c][7]annulene-6-carboxylic acid, cf. ref 8, compd 36. i(Z)-3-hydroxy-2-azaprop-2enoic acid, N is instead of C2. j(2Z,3Z)-2,3-di(hydroxymethylidene)butanedial, torsional angle C=C-C=C -120 deg. k(2Z,3Z)-2,3-di(hydroxymethylidene)butanedial torsional angle C=C-C=C -62 deg. l(3Z,4Z)-2hydroxymethylidene-3-(1-hydroxyethylidene)-4-oxopentanal, torsional angle C=C-C=C -83 deg. m(3Z,4Z)-16 ACS Paragon Plus Environment

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3,4-di(1-hydroxyethylidene)hexan-2,5-dione, torsional angle C=C-C=C -92 deg. n(Z)-3-hydroksy-2azapropenal, N is instead of C2. oHispolon. pCurcumin. qAcetylfilicinic acid. r2,4,6Tri(hydroxymethylidene)cyclohexa-1,3,5-trione. s2,4,6-Tri(1-hydroxyethylidene)cyclohexa-1,3,5-trione.

The unique intramolecular hydrogen bonding in compounds named RAHB is connected with π-electron delocalization and a simultaneous enhancement of the resonance conjugation of the π-electrons. There are numerous evidences44 that each factor affecting the proton donor acidity, the proton acceptor basicity and the total capacity of the conjugated system to form the HB ring will predictably increase its strength. The analysis of the calculated energies of the compounds and their structure relationship shows that both the character and size of the alkyl or aryl groups connected at C1 and C3 have a pronounced effect on the energy of the intramolecular HB. At carbon 2, this effect is insignificant, with energy values going from malonaldehyde 1 with 14.5 kcal/mol to 1,3-dimethyl substituted 5 with 15.2 kcal/mol, 1,3-diisopropyl 9b with 15.4 kcal/mol, 1,3di-tert-butyl 13a with 15.3 kcal/mol and 1,3-diphenyl systems 23 and 24 with 16.0 and 15.8 kcal/mol. For only two structures, namely, 13b and 15, we observe the strengthening of the HB to 17.2 kcal/mol, caused by the steric repulsion of hydrogen atoms at the C2 and C1 substituent (the syn-pentane effect80) “pushing” the oxygen atoms towards each other.81,82 When the rotation of the alkyl groups dampens this repulsion, th EHB value does not significantly differ from the average, even when the bulky tert-butyl group 16, 18, 19 is at C2. When the repulsion cannot be reduced by the rotation, the intramolecular hydrogen bond can be operated at the cost of planarity of the system, which significantly weakens it, as in a non-planar structure of (Z)-3-tert-butyl-4hydroxypent-3-en-2-one with EHB 13.7 kcal/mol8, which have been classified as non-RAHB (see above). The energy of the hydrogen bond significantly changes in molecules with a halogen atom as the substituent, i.e., when F, Cl or Br atoms are attached to the C3 enolic carbon atom, the energy increases up to 23.4, 19.5 and 18.9 kcal/mol, respectively (as in 39, 43 and 47) due to the electronegativity of the substituent but distinctly decreases to 11.1, 11.3 and 11.4 kcal/mol, respectively, in 38, 42 and 46 acid halogenides (C1 substituent). Substitution at the central C2 carbon atom by a halogen decreases the EHB only in malonaldehyde derivatives (13.7 kcal/mol in 40, 13.9 kcal/mol in 44 and 13.7 kcal/mol in 48). In the pentane-2,4-diones, the energy of the HB increases with the van der Waals radii of the halogen atom by 0.6 (41), 2.0 (45) and 2.4 kcal/mol (49) ACS Paragon Plus Environment

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Page 18 of 43

from the fluorine to the bromine with respect to the unsubstituted system. This increase in EHB can be explained by the steric repulsion of the alkyl groups closer to the proton donor and the acceptor oxygen atoms rather than by the halogen electronegativity. In all of the cases in which the molecule contains two halogen atoms (at the enolic C3 and at the C2 carbon atoms), the inductive effect of the halogens on EHB is reduced (compare 39–52, 43–55 and 47–58 ). The second halogen atom at the central carbon atom does not affect the strength of the HB in every halogenide 38-51, 42-54 and 46-57. When carbon atoms C1 or C3 are bonded with strong withdrawing groups (for example, CF3), the energy of the hydrogen bond is virtually unchanged by the negative inductive effect, even for a diverse range of polarizable substituents at the C2 position (methyl, tert-butyl, phenyl, thienyl or furfuryl in the 60 to 71 structures). Unexpectedly, the CF3 group at both the C1 and C3 positions in the same molecule decreases the EHB because it lowers the acidity of the hydrogen atom of the OH proton donor (the hydrogen Mulliken charge in 72 is +0.3956 but +0.4375 in 67). Simultaneously, the basicity of the carbonyl oxygen increases (the oxygen Mulliken charge in 72 is -0.5743 but -0.6549 in 67). The electron donor groups substituted at the enolic carbon atom as amino NH2 74, 76, 78, hydroxyl OH 79a, 79b, 84, 103, 108a and alkoxyl OR 80, 81, 82, 86a, 87b, 88a, 88b, 89 augment the electronic density in the RAHB conjugated system (compare ρBCP, Laplacian and ρRCP in Table S2), thereby enhancing the hydrogen bonding, even up to more than 20 kcal/mol, except for structures in which the enolic C=C bond is common for two HBs in the same molecule, as observed in 103 and 108a. The significant difference between the HB energy for these two “double closed” structures (17.7 kcal/mol and 11.6 kcal/mol, respectively) is caused by the rigidity of the substituted indandione 108a system by the additional HB ring connecting the C1 and C2 carbon atoms. In its less stable “one closed” conformer 108b, the strength of the HB is greater because of the lack of chelation of the HBs. When the extra electron-donor OH group is missing, EHB sharply decreases, as observed in 109 (11.8 kcal/mol) and in 143 (10.4 kcal/mol) because of the anticooperative effect of the cyclopentenedione substitution. Three compounds with resonance-assisted hydrogen bonding in Table 1 (97, 98 and 99) were described in the earlier article8 because of anticooperative effect of two systems: simple saturated HB and RAHB. The result


18

Page 19 of 43

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was the distinct growth of the HB energy of the first one (to 12.5, 11.2 and 9.4 kcal/mol), virtually without changes in EHB of RAHB, comparing with the average. Generally, each alicyclic element between the portions of the molecule that participate in intramolecular hydrogen bonding significantly decreases its strength, as observed in the saturated hydroxy compounds 26, 27, 60a, 61a described earlier8 and molecules with RAHB arising from the cycloalkene group, as observed in 36 (11.1), 95a (12.6), 96a (11.7), and 96b (12.8), and in structures with the similar cycloketo spacer 35 (13.8), 37 (8.2), 108a (11.6 kcal/mol). Comparing the molecules 34 with 35, and 36 with 37 one can see that these cyclic enols exhibit a clear preference for those tautomers in which the double bond is external to the small ring, due to the Mills–Nixon effect45,83and more stable structures have weaker hydrogen bonding. The opposite effect, being also in accordance with Mills-Nixon rule, we observe in the bigger enolic compounds with six-membered external rings, as in the 31-32 or 148-149 pairs. In Table 1, most of the compounds with very strong (above 20 kcal/mol) hydrogen bonding are presented as unnatural, tautomeric structures of the “enol amide,” “enol carboxylic acid” or “enol ester,” that form less stable tautomers, but EHB is considerably stronger. The illustrative examples are pairs 73−74 (+5.3), 75−76 (+5.5), 83a−79a (+8.6), 83b−79b (+6.2), 85a−87b (+3.0), 86b−85b (+4.2), 87a−86a (+0.7), 90b−80 (+4.9), 93b−81 (+3.7), 92c−82a (+5.0), 92d−82b (+4.4), 80c−80b (+4.7), 94c−89a (+4.0), and 94d−89b (+4.4) (the values in parentheses give the difference between the stability of the unstable tautomer (second) in reference to the stable tautomer (first); data not shown in Table 1). For the conceivable four conformers of malonic acid enol form with intramolecular hydrogen bonds, the two saturated stable double carboxylic (24a and 24b)8 structures display a relatively small HB strength (5.2, 5.9 kcal/mol, respectively), but the two unstable, enolic forms with only one carboxylic group display RAHB character with the expected greater energy of H-bonding calculated as 16.1 and 18.2 kcal/mol for 84a and 84b (Table 1), respectively. The cis-trans conformation of the carboxylic groups in the acids plays an important role in the EHB. For the enolic form of the cis-trans dimethyl esters of malonic acid 88a and 88d, the conformation of the methoxy groups has no prominent influence on the HB energy. ACS Paragon Plus Environment

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Page 20 of 43

In acids and esters with a RAHB system (Table 1), the cis conformers are evidently less stable than the trans conformers, with values from 0.4 (86a−86b) to 11.4 kcal/mol (96a−96b) but with evidently higher HB energies, from 1.1 kcal/mol (96b−96a) to 6.1 kcal/mol (88b−88a). In pairs of cis-trans esters, the strengthening of the intramolecular HB in the cis conformers can be explained by net molecule polarization, demonstrated by a beneficial greater difference of charge distribution at the hydrogen and oxygen atoms participating in the hydrogen bond. The distance between both of the carboxyl oxygen atoms, which are shorter by approximately 0.06 Å, in the cis conformers is due to the evident repulsion between the oxygen atoms but, incidentally, increases the participation of their lone pair electrons in the resonance. When the cis conformer is stabilized by cooperation with another hydrogen bond (as in the pairs 110a−110b and substituted indenones 101a−101b), the energy of the HB is enhanced (by 3.7 and 1.3 kcal/mol, respectively). In the RAHB set of compounds, we can observe the interesting series of the functionalized structures (107 – 116) with a carbonyl (aldehyde, ketone, or ester) substituent at C2. In that series, the EHB has an average value (14-16 kcal/mol), except for the structures substituted by the relatively rich in electrons ester groups. For a variety of other C2 substituents, there is no difference among their energies of the intramolecular hydrogen bond. Hydroxypropenal (the enolic form of malonal 1) and its s-trans formyl C2 substituted derivative 111a have the same strength of the HB (14.5 kcal/mol), but its s-cis derivative 111b has an appreciable stronger hydrogen bond (16.3 kcal/mol); the same difference occurs for the 2-acetyl derivatives s-trans 112a and s-cis 112b. All of these structures are planar, and the differences in the EHB values can be explained by the poor mesomeric effect in the s-cis conformers and the advantage provided by the inductive effect of the carbonyl group. There are practically no differences between the HB energies for both the s-cis and s-trans enolic forms of pentane-2,4-dione formyl substituted 113a and 113b, methoxycarbonyl substituted 115a and 115b and ethoxycarbonyl substituted 116a and 116b. In the last six structures, there is no planarity of the extra C2 carbonyl group and HB ring, and the stereoelectronic effects lead to the lower energetic diversity, as found in most of the earlier references.


20

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In compounds substituted at C2 by the electron donating groups OH, OMe, NH2, SPh 120-126, the hydrogen bonding is relatively weaker (14.2-15.3 kcal/mol) than for the substituents NO2, CN 127-131 with an electron withdrawing character (15.3–18.6 kcal/mol), which is consistent with experimental results.77 The electron-withdrawing character of the CN group lowers the electron charge density at the oxygen atoms involved, which in turn causes the enolic proton to be more acidic, therefore increasing the hydrogen bond strength. For structures 131a and 131b, the difference of approximately 2 kcal/mol of the EHB value depends on the syn-pentane effect80 between the C2 and C3 substituents in the 131a conformer. When the substituent withdrawing electrons is bonded to the C3 enolic carbon atom (135-137), the EHB decreases slightly; however, when the extra hydroxyl group bonded with the C2 carbon atom can act with this C3 substituent in an anticooperative manner, each EHB value decreases by 3.8 kcal/mol (135-138a) or by 3.6 kcal/mol (136-139a), respectively, for the given pairs. The turning off of the OH group introduces additional electrons into the enolic system, thus decreasing its acidity, thus making this extra HB impossible to realize, which weakens the primary hydrogen bond by a small amount of 2.0 kcal/mol (135−138b) and 1.6 kcal/mol (136−139b). It is interesting to compare the structure 148 and its saturated, non-planar counterpart (1R,2R)-2hydroxycyclohex-4-ene-1-carbaldehyde (ref 8 compound 27) in which the difference of EHB = 11.5 kcal/mol cannot be ascribed solely to the non-planarity of the second structure. By comparing the EHB values in the enolic forms of the acetyl substituted indandione 109 (11.8 kcal/mol) and cyclopentenedione 143 (10.4 kcal/mol) with cyclopentanedione 147 (16.3 kcal/mol), it does not appear that these serious differences are introduced solely by geometry but rather by the conjugation of the outside C=O and C=C groups. In the acetyl (147-146) and formyl (145-144) cyclopentanediones, the additional carbonyl group conjugated with the RAHB system plays a slightly destabilizing role for the EHB of 0.7 to 1.9 kcal/mol being trans to the enolic C=C bond. In that case the MillsNixon rule does not apply. For the compounds 150 to 167 (Table 1), the small changes in the energy of the hydrogen bonding (0.4 kcal/mol) are not precisely correlated with the diversity of the geometry or the electron density neighboring the RAHB systems. The EHB in curcumin 154 (15.3 kcal/mol) is nearly the same as in the other heptatrienones 150 and 151 and in the enolic form of acetylacetone 5 (15.2 kcal/mol). In addition, the series of the 1,3-diphenyl ACS Paragon Plus Environment

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Page 22 of 43

substituted RAHB system provides no diversity, although our EHB values are very close to the experimental values obtained25 by the dO···O method taken from crystallographic measurements. The exception is the structure 169, where 4-methoxy substitution in the phenyl ring of H-bonding donor increases its strength by 2 kcal/mol. Triformyl-170 and triacetyl-171 cyclohexanetriones in their enolic forms show very strong HBs (18.3 and 21.9 kcal/mol) probably because of total coplanarity of the all three resonance assisted hydrogen bonds existing in both molecules.

Table 2. Table 2. MTA Energy of Intramolecular Hydrogen Bond (EHB in kcal/mol) for Aliphatic α-Alkilideneγ-Hydroxy β,γ-Unsaturated Carbonyl Compounds

no

R1

R2

R3

─EHB

172

H

H

H

16.7

173

Me

H

Me

17.2

174

Me

NO2

Me

18.0

175

CH2Cl

H

CH2Cl

16.4

18.3984

176

CHCl2

H

CHCl2

18.0

16.6084

─EHB lit

When a five-membered C2-C3 cyclic spacer (with two additional double bonds, creating ten π electrons systems) is included in the HB ring of pentadiene, as in the substituted fulvenes (172 to 176) the hydrogen ACS Paragon Plus Environment

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bonded molecules resemble the aromatic structure of azulene. The energy values of 16.7 kcal/mol to 18.0 kcal/mol, which are close to those in typical six-membered resonance-assisted hydrogen bond structures, are therefore not surprising. Because the RAHB can be treated as a quasiaromatic system,7,19 therefore, the structures from Table 2 are recognized as RAHB systems (named by Gilli3 as R7-RAHB). Moreover, when in two RAHB compounds 36 (EHB=11.1 kcal/mol) and 37 (EHB=8.2 kcal/mol) the saturated external ethano bridges are replaced by the double bonds, the additional pair of π electrons deprives them of RAHB character (calculated EHB is 6.9 and 0.6 kcal/mol respectively), in spite of the full planarity of both enolic structures (see 2-(hydroxymethylidene)cyclobut-3-en-1-one with EHB = 0.6 kcal/mol and 2hydroxycyclobuta-2,4-diene-1-carboxyaldehyde with EHB = 6.9 kcal/mol, see the end of Table S1 and Table S2.

3.2 Chelated β-ketoenols Table 3. MTA Energy of Hydrogen Bond (EHB in kcal/mol) for Chelated Aliphatic β,β'- Dihydroxy α,β, α’,β’Diunsaturated Ketones.

no

R2

R3

R2'

R3'

─EHB

177

H

H

H

H

13.5

178

H

F

H

F

18.3

179

F

H

F

H

13.9

180

H

Me

H

Me

13.8

181

Me

H

Me

H

14.7


23


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182

Me

Me

Me

Me

13.5

183

H

Ph

H

Ph

13.8

184a

a

Me

a

Me

15.9

185b

b

H

b

H

12.9

186c

c

Me

c

Me

11.7

Page 24 of 43

a 2 2'

R R -C(O)-O-C(O)-.b(2Z,3Z,5Z,6Z)-2,3,5,6-tetra(hydroxymethyl)cyclohexane-1,4-dione.c(2Z,3Z,5Z,6Z)2,3,5,6-tetra(hydroxyethylidene)cyclohexane-1,4-dione

The chelated enolic structures summarized in Table 3 compared with the mono enol parent compounds from Table 1 show the predicted decrease of the hydrogen bonding strength, with a difference ranging from 1.0 kcal/mol (177−1) to 5.1 kcal/mol (178−39). This anticooperative effect85 is the natural consequence of the limited delocalization of the ten π-electrons engaged formally in two RAHBs (two hydrogen bonds are in position to compete for the two lone pairs of the same sp2 carbonyl oxygen atom). The HB energy in the enol of 3,5-diacetyloxane-2,4,6-trione 184 (EHB =15.9 kcal/mol) is greater than that in the trione 180 enolic form (EHB = 13.8 kcal/mol), most likely because of a conjugation with the anhydride spacer at the C2 and C2’ atoms in the relatively flat molecule.

4. DISCUSSION The energies of intramolecular resonance-assisted hydrogen bonds calculated by the MTA method shown for 229 conformers of 186 compounds in Tables 1-3 were correlated with their geometrical, spectral and topological calculated properties. It was found that some energy parameter relationships can be described by the linear or quadratic equation. The curves describing this dependence have a different coefficient of regression than for the saturated structures; thus, the RAHB subset can be clearly distinguished as the set of the intramolecular hydrogen bonded structures. The correlations of the EHB and the geometrical parameters described below are not so strict as for saturated systems, the other parameters characterizing the HB strength are correlative but are not always sufficient to ACS Paragon Plus Environment

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quantify the HB-interactions.15,26,37,39 In fact, only the length of the covalent O-H bond can be used as the measurement of the strength of RAHB.

Figure 1. MTA intramolecular hydrogen bond energy as a function of hydrogen bond length H···O for structures with RAHB (the exceptional structures are marked as triangles). Figure 1 presents the quadratic correlation between the H···O (rHB) distance of the intramolecular H-bond and the HB energy (EHB). For the structures with RAHB the H···O (rHB) distance is primarily the effect of the rigid geometry of the quasiaromatic structures stabilized by the conjugation of π electrons that are occasionally distorted (37, 37), also the cooperative hydrogen bonding diminishes EHB in the hydroxyindenone structures 101a and 106 (triangles). The other (R7-RAHB, ten π-electron aromatic, see above) cooperation is responsible for the shorter hydrogen bond in substituted fulvenes 173 - 176. As we mentioned, the calculated HB energy strongly depends on the electronic effects of the substituents, and when these effects are significant, the EHB is lower than the rHB would indicate, as observed in 131a and 131b (a CN group at C2) and in 78 (a NO2 group at C2) (Table 1). When the fluorine and electron donating groups as OH, OR, NH2 are attached to the enolic carbon atom (C3), the strength of hydrogen bonding increases because of the growth of electron density in whole coupled system (compounds 39, 52, 76, 79a and 88b respectively).


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Figure 2. MTA intramolecular hydrogen bond energy as a function of hydrogen bond angle O-H···O for structures with RAHB (the exceptional structures are marked as triangles). Figure 2 shows, that the six π-electron resonance-assisted hydrogen bonding in six membered ring forms when atoms of O-H group and atom of carbonyl oxygen forms the angle from 136 to 156 degrees. For systems with ten π-electrons with seven membered ring (172-176) this angle grows to 170-175 degrees. It is evident, that the angle of each of the intramolecular hydrogen bonds depends strongly first of all on the membering of the HB ring formed. When molecules with substituents that introduce rigidity to the system (triangles of compounds 37, 107, 109, 143, 172-176) are omitted, for structures with RAHB the dependence is quadratic.

Figure 3. MTA intramolecular hydrogen bond energy as a function of the length of covalent bond O-H, for structures with RAHB, (the exceptional structures are marked as triangles). Although experimentally difficult to measure, the elongation of the O-H bond of the O···H-O system is one of the “signatures of H-bonding”. Here for RAHB systems (Figure 3) there is good linear correlation between the calculated HB energy and the O–H covalent bond length (dOH). However for special case of 2-nitrodimalonamid


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78 (Table 1) the calculated energy of the HB is distinctly lower than expected from O-H distance because the C2 nitro substituent strongly diminishes the electron density, (therefore modify the resonance of the RAHB system; moreover the NO2 substituent creates two additional anticooperative hydrogen bonds with both NH2 groups. In the molecules 172-176 with the 7-membered, ten π-electron fulvene type RAHB system the MTA calculated EHB values are also not directly connected with the elongation of dOH (see above). Compounds 37, 107 and 143 show EHB weaker than expected from dOH because of external 4- and 5-membered stiffening ring.

Figure 4. MTA intramolecular hydrogen bond energy as a function of oxygen atoms distance O···O, for structures with RAHB (the exceptional structures are marked as triangles). Figure 4 shows, that the O···O distance (dO···O), often used for the estimation of the strength of the intramolecular hydrogen bond, for compounds with C=C-C=O resonance is not an adequate and useful tool for the same reasons, as was earlier interpreted for the length of hydrogen bonding r(HB). Moreover at Figure 4 (triangles), we can observe that the same distance between oxygen atoms d(O···O) occurs for compounds quite different both geometrically and electronically. For example the diversity of EHB achieves above 7 kcal/mol (39 with 99 and 76 with 131b).


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Figure 5. MTA intramolecular hydrogen bond energy as a function of ν(O-H) frequency, for structures with RAHB (the exceptional structures are marked as triangles). From occasionally double νOH frequencies from the calculated IR spectra of the analyzed compounds only the higher frequency was considered because typically it was more intense. The linear correlation of the EHB and the νOH is worse than for non-enolic structures.1 Here (Figure 5), in some cases, the molecule made more rigid by the anticooperative hydrogen bonds or outer rings (triangles for substituted dibenzoannulene 99, cyclopentenes 107, 143, and cyclobutane 37) forms a hydrogen bond that is weaker than expected from νOH. There are also some deviations for the analyzed ten π-electron fulvenes 172-176. The calculated EHB does not correlate with the frequency of the C=O frequencies for compounds with RAHB (see Figure S1). According to the theoretical analysis of electron density AIM theory the strength of a hydrogen bond can be also quantified by looking at the electron density at the corresponding bond critical point (ρBCP), its Laplacian ∇2ρBCP and at the ring critical point ρRCP. After full optimization (see Methodology) for each of 229 structures the AIM analysis was carried out on the MP2-derived wavefunctions, and for each structure was found the bond critical point (ρBCP). It seems that the characteristics of ring critical points parameters, particularly of electron energy densities, should reflect some properties of the ring systems, for example, the quasi-aromaticity of a RAHB ring.


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Figure 6. MTA intramolecular hydrogen bond energy as a function of HB topological parameters: electron density in BCP (A), and electron density in RCP (B), for structures with RAHB (the exceptional structures are marked as triangles). The energy of 229 intramolecular resonance-assisted hydrogen bonds presented in this study is correlated with the electron density at the bond critical points ρBCP in quadratic relations. Figure 6A demonstrates that for certain RAHB compounds 39, 52, 76, 79a-b, 80, 82a-b, 88b (triangles) in Table 1) with the electronic rich substitution (OH, NH2, OCH3, F) at the C3 carbon atom, the EHB increases faster than the electron density in the critical point. For the other compounds: 78, 131a-b with electron-withdrawing C2 substituent or strongly stiffed 36, 37 (Table 1), 173 and 174 (Table 2) the strength of the HB is smaller than that expected from the ρBCP value for the reasons presented by the rHB discussion and it cannot be used for the estimation of the energy of the hydrogen bond. The EHB for saturated compounds8 changed proportionally to the Laplacian ∇2ρBCP value at the H····O bond critical point with a quadratic trend, however this dependence nearly disappears for RAHB structures (see Figure S2). ACS Paragon Plus Environment

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Although the dependence between the energy of the intramolecular HB and the electron density at the ring critical point ρRCP was unspecified for saturated compounds8, with various substituents and size of the bonded rings, reveals an average quadratic correlation for more investigated compounds with RAHB. The 3-hydroxy-2azaprop-2-enoic acid 100 and aldehyde 142 (Table 1) contain the nitrogen atom instead of the C2 atom in the six-membered HB ring, therefore having a different ring critical point than the other structures. Also, two structures 36 and 37 show hydrogen bond weaker than expected from ρRCP because of external rigid cyclobutyl bridge. The five fulvenes 172-176 (Table 2) described above, with an increase of π-electron delocalization also do not belong to the curve in Figure 6B. The calculated absolute shieldings σ (isotropic tensors) of the proton engaged in the intramolecular hydrogen bonding for each of investigated structures were transformed into chemical shifts δH. The resonance in RAHB systems individually modified by the substituents makes the magnetic response of the proton participating in the hydrogen bond more complex and therefore diminishes the regression coefficient. This effect is connected also with the influence of ring current present with the lateral rings of this molecular system.86 So the correlation between the EHB and δH in RAHB systems is insufficient to estimate the energy of the intramolecular hydrogen bond (Figure S3). Figures 1 to 6 confirm that the energies of the resonance-assisted intramolecular hydrogen bonds correlate with a variety of topological, geometrical and spectral parameters of the bonded molecules and can be described, respectively, by the linear or quadratic equations and that these dependences are distinctly different for compounds with the simple, saturated HB. A proportionality between EHB and the value of the HB angle φ(HB) occurs only for RAHB systems. In the RAHB systems the hydrogen bonding is clearly stronger (in the range 8.2 – 26.3, typically 14 - 16 kcal/mol) than for saturated compounds (1.4 - 13.7 kcal/mol1). The RAHB, the same as the aromaticity, is a collective phenomenon and various criteria do not always give the same result. A variety of criteria should be used in the discussion of this term in any particular subject of investigation.

5. CONCLUSIONS ACS Paragon Plus Environment

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A novel method proposed for the estimation of intramolecular hydrogen bond energy based on MTA method was earlier8 verified on some intermolecular interactions and on over 150 structures showing the common intramolecular O-H…O=C hydrogen bonding. During this simple computational method an ab initio procedure (after a density functional theory preliminary optimization) and single point energy calculations is applied. For the calculation of the strength of HB the geometry optimization of molecule is needed, followed by the computation of single point energy of three tailored fragments without HB. In the RAHB systems the hydrogen bonding is clearly stronger (8.2 – 26.3 kcal/mol) than for saturated compounds (1.4 - 13.7 kcal/mol1) and we confirm that the strong intramolecular hydrogen bond combined with the energy gain provided by the π-electron resonance might contribute to the excellent stability of the keto-enol molecules in the gas phase. Calculated EHB values show correlation with the both O-H···O length and angle, length of O-H covalent bond and d(O···O) distance, stretching frequencies vOH, and the value of the electron density in both bond critical point ρBCP and ring critical point ρRCP. The reported results reached with our method attain the statistical significance and allow properly interpret the weakening (or strengthening) of the intramolecular hydrogen bond by substituent on the ground of general theory for organic compounds. The strength of RAHB depends also on the steric accessibility of the donoracceptor environment and the cooperativity with other HB. We found, that even subtle, structural and stereoelectronic effects are well reflected by the calculated hydrogen bond relative energies. We are of the opinion, that the method, being quite general, can be applied also to aromatic systems and, we hope, to increasingly strained five membered rings and to other hydrogen bonded systems, including N-H···O and O-H···N in amides and peptides. ASSOCIATED CONTENT Table S1. Systematic Names of Examined Compounds and Calculated Energy of the Intramolecular Hydrogen Bonds (kcal/mol). Trivial Names in Parenthesis. Table S2. MTA Energy of Intramolecular Hydrogen Bonding EHB [kcal/mol], Length of the HB as H···O (rHB) [A], Angle of the HB as O-H···O (φHB)[deg], Length of the O-H Bond (dOH) [A], Distance Between ACS Paragon Plus Environment

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the Oxygen Atoms as O···O (dO···O) [A], Frequency of O-H and C=O Stretching [cm-1], Electron Density in the Bond Critical Point (ρBCP) [au] and its Laplacian, Electron Density in the Ring Critical Point (ρRCP) [au] and HNMR Chemical Shifts δH [ppm] Calculated for Structures 1 – 182. Structures 183 and 184 serve to compare them with 36 and 37, are not RAHB and were not previously reported. Table S3. Energy of Resonance Assistant Hydrogen Bond (EHB kcal/mol) Calculated for Selected Compounds by MTA Method with H or F Substitution. Table S4. Energy of Intramolecular Hydrogen Bond (EHB kcal/mol) Calculated for 3-Hydroxyprop-2-enal by MTA Method with H or F Substitution. Figure S1. MTA intramolecular hydrogen bond energy as a function of ν(C=O) frequency for structures with RAHB. Figure S2. MTA intramolecular hydrogen bond energy as a function of Laplacian in bond critical point for structures with RAHB. Figure S3. MTA intramolecular hydrogen bond energy as a function of chemical shift of H-bonded proton. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT This study was supported by Poznan University of Technology 03/32/DSPB/0500. The authors also acknowledge Poznanskie Centrum Superkomputerowo-Sieciowe, Poznan, Poland, for computational time. REFERENCES


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Intramolecular O-H···O=C Hydrogen Bond Energy via the Molecular Tailoring Approach: Part II RAHB Structures


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Danuta Rusinska-Roszak*,


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Intramolecular OâHÂ·Â·Â·O C Hydrogen Bond Energy via the Molecular

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