MO investigations on lignin model compounds

Advances in Molecular Relaxation and Interaction Processes, 14 (1979) 3’7-46 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

MO INVESTIGATIONS ON LIGNIN MODEL COMPOUNDS IX. A PCILO study of the hydrogen bond O-H...0 anhO-H...N type formed by phenol with some proton acceptorsX

Milan Remko

Pulp and Paper Research Institute, 890 20 Bratislava, Czechoslovakia

(Recieved 11 April 1978)

ABSTRACT A systematic PCILO study has been carried out on the intermolecular hydrogen bond formed by phenol with proton acceptors such as acetone, ether, p-benzoquinone,N,N-dimethylfonnamide,trimethylamine,pyridine, acetonitrile and acrylonitrile. The complexes studied form hydrogen bonds with energy intervals of 20 - 45 kJ mol-' at distances R = 0.265 - 0.300 run 0 1..Y (Y=N,O). For the system phenol-pyridine both the 1:l complex PhPy and the 2:l complex Ph2Py were also studied. The calculated hydrogen bond energies are compared and discussed with experimental data from the literature. The calculated hydrogen bond and O-H stretching force constants are in range expected.

INTRODUCTION The hydrogen bond plays an important role in the formation of molecular complexes and stabilizing the conformations of macromolecules. Particular interest has been shown in biologically important molecules [1,2], both experimentally and theoretically. So far not much effort has been devoted to the study of the hydrogen bonds in lignins to provide information on the utilization of this natural aromatic material. The role of hydrogen bonding in the study of the solubility and network structure of lignins is well known [3].

x - for Part VIII see ref. 38

37

38 Lindberg et al, [4-lo] carried out experimental investigationsof the hydrogen bond in lignin model compounds and in the macromolecule of lignin. Theoretically, using the semi-empiricalCNDO/Z method ull the hydrogen bond of some lignin model compounds has been studied

DZ-lo]

.

In this work we used the PCILO method [20,21] to study the intermolecular hydrogen bond formed by phenol, basic representativeof the p-hydroxyphenyl type end group in lignin, with proton acceptors such as acetone, ether, p-benzoquinone,N,N-dimethylformamide,trimethylamine,pyridine, acetonitrile and acrylonitrile, The PCILO method was recently successfully used for the study of the hydrogen bond [22-251, because the combination of the CNDO/Z parametrization with perturbation theory and the inclusion of the configuration interaction in the PCILO method leads to a distinct improvement of the results for hydrogen bonded complexes [25].

CALCULATIONS For the calculations of the equilibrium geometry, energy and electron structure of the hydrogen bonded systems studied the well known semiempirical PCILO method [20,21] has been used with original parametrisation. The geometry of the systems studied has been optimalised with respect to the following parameters (Fig. 1): (i) distance R

(Y=N,O), (ii) distance rol.B9assuming the O-H,.,Y bond

is linear, Al?'&tems

studied were assumed to be planar with Cs syrmnetry.

Hydrogen bond energy was defined as the difference between the total energy of the isolated molecules and the total energy of the hydrogen bonded systems, The calculationswere carried out with experimental geometry [26] of the monomers on a Siemens 4004/150 computer at the Computer Centre of the Komensky University, Bratislava using the QCPE 220 programme.

EQUILIBRIUM GEOMETRY AND ENERGY OF THE WDROGEN BOND In Table 1 the equilibrium geometries, hydr,ogenbond energies and dipole moments calculated by the PCILO method are listed for the systems studied, together with experimental energies. The calculated PCILO lengths (Y=N,O) are calculated longer than that calculated by the CNDO/Z Ro,,,Y method for similar hydrogen bonded phenol complexes [15-171. In most of

39

Fig.

1

Model of hydrogen bond in the systems investigated

0,286

. , .N CCH3) 3

,.,pyridine

,.,CH3CN

...CH2=CHCN

5

6

7

a

.

0,271

..,@H3)2NCOH

4

0,271

0,270

0,309

0,271

,..p-benzoquinone

3

0,264

0,271

.1( a31 20

phenol,,, CCH-,)2C0

1

0.106

0,106

0.106

0,105

0,105

0.105

0,105

0,105

R 0,,,Y' lIrn rosy urn

2

System

44.47

45.14

20.02

23,78

25,74

25.20

38,37

molcl

24.24

26,04

21.10

15.88

19.56

W

la.97

18.4-19.81

23,4-33,44

k.J~101"' Ez?

26,33

Em,

45

39,44

39,43

42

39

41

40

39

Ref.

Equilibrium geometries, energies and d&pole moments of the hydrogen bonded phenol complexes calculated Sy the PCfLO method

No,

TABLE 1

1.44

1.35

1.25

1.15

1.14

0.66

1.78

1.21

29 !.lxlO,C.m

b

41

the systems studied the calculated and experimental values of the hydrogen bond are in a good agreement. Difficulties are raised by comparison of theoretical results (correspondingto the gaseous state) and available experimental data (correspondingmostly to the condensed phase), The experimental studies in the vapour phase were only performed for simple alcohols [27-311, some dimers of first and second row hydrides and other simple molecules [2,32,33], but no data existed for phenols such data until1 now,

Thermo-

dynamic data for hydrogen bonded complexes which have been studied in both vapour and condensed phases indicate that these complexes are more stable in the vapour phase [28]. According to the Table 1 the largest difference between theory and experiment was observed by hydrogen bond of the phenol-acetonitrileand phenol-acrylonitrilesystems. Experimental studies indicate that nitriles are weak proton acceptors [1,2]. Recent experimental [34] and the CND0/2 [35] investigationsby Gramstad et al. have shown that, so far as the existence of planar parallel-antiparalleltrimers of the acetonitrile held together by a strong dipole-dipole interaction is concerned,a hydrogen bond of the O-H ,*. IT type with a lower association energy is most probable. Both 1:l (PhPy) and 2:l (Ph2Py) complexes in the case of the phenolpyridine (PhPy) system have heen studied, By Ph2Py complex, the angle c-o...H-0 was held at 120' [18]. The lengths R. o and ro_H have been *.t varied (Fig. 1) at the equilibrium geometry of the phenol-pyridine complex, Our PCILO calculations have shown that hydrogen bond O-H,,,0 with energy of 21.86 kJ mol-' is more stable than that O-H,,,N of the PhPy system by shorter lengths R.

o = 0.266 nm and r. H = 0.105 nm. Baezer et al. [36] .I, studied, by calorimetry, the hydrogen bond of the phenol-pyridine system in detail. They estimated the molar enthalpy of formation of PhPy to be 27.0 k.lmol",

In more concentrated phenol solutions some Ph2Py appears to

be present, The enthalpy of formation of this species is at least 34.6 W -1 mol 1 Our PCILO calculations for this complex represent the value of 41.88 kJ mol-'. Some insight into the nature of the hydrogen bond can be gained from the energy partitioning. The total hydrogen bond energy, EDP, in the PCILO formalism can be written as [25]: = hEo + EHB

AE2 -tAE3

(1)

where AEi is i-th order contribution to the hydrogen bond energy. The second order contribution, AE2, may be further subdivided:

42

AE2

= Am1 + Am2 + Ad1 + Ad2

(2)

where Am1 is polarization energy, Am2 the delocalization energy, Ad1 the intra-bond correlation energy and Ad2 the inter-bond correlation energy. In Table 2 the results of the partitioning of the hydrogen bond energy into the various contributions are listed for the systems under study at the equilibrium distances of the complexes. AEo is in all systems negative (i.e. destabilizing). The second order contribution, AE2, is positive and large (i.e. stabilizing) and comes mainly from delocalization contribution Am2. The intra-bond correlation energy, Am,, is small and destabilizing on the contrary, the inter-bond correlation energy contribution Ad2, is stabilizing. AE3 is in all systems negative.

STRETCHING FORCE CONSTANTS The stretching force constants for stretching of O-H and hydrogen bond, O-H .,.Y, (Y=N,O) of the systems studied are given in Table 3.

Force con-

stants are evaluated numerically from potential energy curves by a polynominal fit. For monomer phenol stretching force constant, FOH, has been -1 and decreases on hydrogen bond formation found the value of 18.41 N cm (Table 3). Purcell and Drago [37] solved vibrational secular equations for the corresponding force constants of the system phenol-trimethylamine, -1 considering only the fragment 0-H...N. They found the values 7.66 N cm , -1 for phenol monomer, phenol adduct and hydrogen 5.02 N cm-' and 0.47 N cm bond stretching force constants respectively.

CHARGE SHIFTS AND CHARGE TRANSFER In Table 4 the charge shifts and charge transfer of the systems under study are presented, CSX represents charge shift on phenolic oxygen, CSY charge shift on the proton acceptor nitrogen or oxygen, CSH - charge shift on hydrogen bonded hydrogen and CT - total charge transfer, i.e. the total number of electrons gained by phenol, According to Table 4 the charge density on the oxygen of the proton donor group increases in all cases, and the charge of the proton acceptor atoms oxygen and nitrogen upon hydrogen bond formation decreases. The hydrogen bonded hydrogen has a positive character and its charge density is lowered due to the formation of the hydrogen bond.

In all cases studied,

43

TABLE 2 Energy terms of the perturbation development in studied hydrogen bonded phenol complexesa

System

AE0 -58.89 -87,19 -49.32 -53.50 -49.99 -33,81 -72.69 -75,82

System

AB2

9

-4 Cl0 -4 Cl0 -4 Cl0 -4 Cl0 -4 Cl0 -3 cl0 -4
Ad2

Am2

Ad1

112.27

-1,63

8.15

178.61

-1,17

10.86

97.64

-1.21

8910

104,87

-1.46

6.98

121,Ol

-0.62

8,40

63.57

-0.50

9.99

127.61

-0,62

9.99

130.79

-0.66

10.45

ABo +

AB2

+AE

3

EHB

118.73

59.90

233.56

26.33

188-30

101.11

-62.74

38.37

104.53

55.21

-30,Ol

25,20

110.39

56.89

-31,15

25.74

128,79

78,80

-55.02

23.78

73.06

39.25

-19,23

20.02

136.98

64.29

-19,15

45.14

140.58

64.76

-20,29

44.47

a P all energy terms in kJ mol-1

TABLE 3 O-H and O-H ..,Y stretching force constants in N cm-1

System

F O-H

FO-H,,,Y

14.65

0.42

14.96

0.94

14.33

0.76

15.59

0.30

14.35

0.48

15,43

0.61

13.35

0.49

14.03

0,98

TABLE 4 Charge shifts and charge transfer in studied hydrogen bonded phenol a complexes

csX

-0.0373

0.0474

0.0345

0.0449

-0.0851

0.0502

0.0362

0.0415

-0.0656

0.0401

0.0241

0.0378

-0.0716

0.0436

0.0309

0.0409 0.0533

-0.0821

0.0384

0.0328

-0,0372

0.0219

0.0115

0.0255

-0.0964

0.0442

0.0511

0.0617

-0.0992

0.0457

0.0518

0.0633

a - negative CS values indicates excess of electron density compared to monomer value,

45 charge transfer from the proton acceptor to the proton donor has been found. However, this magnitude has no correlation with the hydrogen bond energy.

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