Isomerization of azomethine- and azo-compounds and their protonated species

CHEMICAL PHYSICS lU973)

340-347. NORTH-HOLLAND

ISOMEFU~ATION

P’JBLISHlNG

OF AZOMETHINE-

AND AZO-COMPOUNDS

AND THEIR PROTONATED G. MERl%Yl Dqarrmenr

COMPANY

SPECIES

and G. WETTERMARK

o/,Z~ysical Chemistry, Royal Institute of Technology SIOO 44 Sfoekholm 70, Sweden

(KTN),

and B. ROOS Institute of Thcorerical

..

Physics, University

of Stockholm.

Stockholm,

Sweden

Received 30 hlarch 1973

Barriers of rotation and inversion, respectively, have been calculated lor the species H2C=NH (I), H,C=NCHS (II), NH=NH (III), NH=NCHj (IV) and their protonated specicr For any unprotonated molecule the bvrier of invcrsion is consistently lower than the barrier of rotation. The inversion barriers arc: 27.6 (I), 23.8 (II), 51.9 (III) and 46.1 (IV) kgllmole. In the case of azomethinc spccics, protonalion results in an increased rotational barrier (from SO.8 to 74.7 kcd/molc for 11). In the GLS of azo species barriers of inversion arc lowered on protonalion (from 51.9 to 30.1 for II and from 46.1 to 24.4 kti/molc for IV). AU bvrica arc given with reference to the mmr+omer (azo). Proton zffmities for the azomethine species are higher than those of the corresponding azo species’(223.3 for I, 199.9 kcal/molc for II).

1. In!roduction Experimentally

it is well established that borh may undergo cis + fmns-ic.omerization In solution [l-3 J. Both azome. thhe- and azo-compounds participate In reactions where ?ereospecificity is required 14, S]..ln many cases, a reaction can only proceed via the &-form,

azomcthine-

and azo-compounds

.which is less stable than the mans-form. The rate:determinlng step In many reactions in which azomethine- or azo-compounds are involved, is often the rruns * cis-isomerlution. Insuch instances, it is of primary interest to establkh’what eHe& acid cataly sis may have on the reaction rate. From the purely theoretical point of view it is of great interest to investigate the’influence of protonation on the different, modes of molecular tsomerizatlon and an ’ attempt Is made here tb correlate these changes wIti.. ,.&ang& in blectronic dlstrlbutions and bond strengths.. Some experiments have been done on azo-compounds

in order to study the effect of acid catalysis on the rate of cir + rmns-isomerizatton [2,3]. The conclusion has been drawn that protonation enhances the rate of isomerization. On the other hand, data are more scarce as regards the possible acid catalysis of azomethlne isomcrizatlon. As is well-known, GN-compounds tend to hydrolyse In aqueous aclciic media [6]. Because these two effects are difficult to separate, any experimental effoit to estab lish the effect of the acid on the isomerlzatlon rate of azomethine-compounds is liable to be Mated. The present calculations serve therefore two goals: (i) To explain the observed effect of acid catalysis on the isomerizatlon of azecompounds (ii) To investigate theoretically the possible effects of protonatlon on the nte of lsomerizatlon of the C=N-compounds for which the Interpretation of experlmental t$&ngs is likeli to be ambiguous Both azom&ineand azo-compoundshave been the ..subject of various ab lnttlo calculations. Lehn and co-

G. hferbyi

er al, Isomerizarion of azomerhine. and azcwompounds

workers [7-9] have performed detailed ab in.Itio calculations on the model substance H2C=NH. Their calculated values for the barriers of inverdon and rotation, respectively, are 29.9 kcal/mole and 57.4 kcal/mole. Comparison of the above values clearly shows that lsomerfzatton of normal azomethine comRounds proceeds via an invertomer rather than a rotamer. Furthermore, Levy and co-workers [9] have made a thorough investigatfon of the origin of inversion banlers of H,C=NH in terms of localized SCF bond orbit&. Wong [lo] and co-workers and L&m [7] et al. have performed ab h-&lo calculations on the a.~ocompound HN=NH. Both works confirm that the puns-form is more stable than the &form_ Lehn calculated the barriers of inversion and of rotation, respectively and obtained the values 50.1 and 86.1 kcal/mole. Wong et al. performed a detailed optimization of bond lengths and angles of the HN=NH compound. Furthermore, they discussed the possible existence of H2N+=N-, which has been suggested in the work of Hiinig and co-workers [S] . Their calcuiated values for force constants and IR frequencies of HN=NH were in good agreement with the experiments of Rosengen and Pimentel [ 1 l] _Moffatt [ 121 has performei some calculations on the protonated H2C=NH. However, he used a rather limited basis set in order to test whether the set was sufficient for the case. For this reason, his calculated proton affinities are not supposed to be quantitative.

2. Methods of calculation

1

The present ab Wtio calculations have been performed by means of the program REFLECT [ 131. The basis set chosen consisted of Gauss-type functions which were optimized with respect to individual atoms. Four s-type orbltals were employed for the description of the hydrogen atoms and seven s-type and three p-type orbitals for the descdption of the carbon and nitrogen atoms. This basis set, called (7,3/4), was then contracted into a (4,212) set (double zeta). All exponents and contraction coeffidents were taken from the work of Roos and Siegbahn r141-

341

3. Results

Bond lengths and angles for H$=NH and H,C=NCH, were taken from the experhnental’iesuits of Sastry et al. [ 1S] tith the exception of: the C-H-N angle, which was taken from the optimization of Lehn [7,8]. The N-H bond length in HN=NH is the same as tn L&us work [7]. Furthermore, we employed the optimized angles and the N-H distance for aLl conformers of HN=NH according to Lehn et al i71- The same geometry was used for HN=NCH3 with proper consideration for the N-CHJ dtstance. The latter was taken from Sastry’s findings on the H2C=NCH3 molecule [ 15 1. In add&

Fig. 1. The electrostatic potentials in the plane of the H2C=NH molecule. The plot is obtained by connecting points for which the rxpression

V(q) = -

all

J

has the same value. Here p stands for the electronic density, Za for the charge on nucleus ct. rtiand rai denote the d&ance of point i from electron 1 and nucleus P respectively. Solid lines cicnote positive and dashed lines negalive potentials. Points with zero ovaall potential arr marked by dotted lines. l-he distances on the coordinate axes are given in aromic units. Also, tic KU&IS isopotential lines are given in atomic energy units. comprising the levels -0.14. -0.10, -0.8. -0.6. -0.4, -0.2,0.0,0.1,0.2,0_4.0.8,0.16,0.37.0.64,1.28, 256 au. The minimum in the potential well b -0.1493 au mrrespondtng ICI-93.7 kallmole.

342

G. Men?nyi it aL, Isomerization

:

of azomethine-

‘.

and aza-com$otinds

Table 1 Energies and bond lcngrhs for thc’azomctinccompounds Compound

C=N

T&l

Ranier

length 6-Q

energy

(kclllmole)

(au)

H2C=NH invertomcr rotamcr

1.26 1.235 1.275

- 93.8965 - 93.8522 - 93.8039

27.8 i 58.1 r

H2C=N+HI

1.28 1.38

- 94.2522 - 94.1381

76.6 r

rohmer

- 1328789

H1C=NCH3 invertomer rolamer

- 1328409 - 132.7864

23Ai 58.0 r

H2C=N+HCH3 rotamer

- 133.2529 - 133.1339

73.1 r

Proton affmity (km.l/mole)

n-orbital energy (au)

223.3

- 0.445 - 0.429

234.7

- 0.412

i = inversion, r = rohtion. Table 2 Total charges and dipole moments

for the azomcthine-compounds

Total charges on different atoms Compound H2 ‘=C=N ‘Ii” H” invcrtomer rohmcr

H4 3 H

HZ ‘C=N+( H” rotamcr

HZ ‘C’=N H’/

HS ‘cy

N

C

H’

HZ

H3

Dipole moment (dcbye)

- 0.52

- 0.09

0.16

0.19

0.27

229

- 0.72 -0.73

0.05 0.08

0.14 0.16

0.14 0.16

0.38 0.33

0.32 2.48

N

C

H’Z

fp.4

- 0.63

0.12

0.32

0.43

0.33

- 0.07

0.35

0.32

0.44

1.95

N

c’

c?

H’

H’

HL4

Hs

- 0.55

- 0.07

- 0.28

0.15

0.19

0.18

0.20

208 a)

0.03 0.11

-0.15 - 0.20

0.13 0.15

0.13 0.15

0.19 0.16

0.17 O.lI?

0.81 208

c’

c2

H’

H2

H314

H5

H6

0.12

- 0.31

0.31

0.31

0.27

0.27

0.43

1.35

0.40

- 0.27’

0.31

0.31

‘0.23

.0.28

0.43

1.11

kH3”

invertomer

-

0.70 -0.72

rotamer -. -I?

N -

H6 ‘C’

=N+

Hv

- 0.68

\
H’ rotamer .a$&mcatA:

- 0.92 ‘.. I.53 * 0.020D [li].

345 Table 3 Energ& and bond lengths for the h-compounds Compound

N=N length

Total (A)

energy (au)

Barrier rkcal/mdc~

RODJll

7POrbild

aftidly &cal/moIe)

energy (au)

199.9

- OS 14

HN=NH (hens)

1.23

- 109.8175

HN=NH (cis) invertomer rotamer

1.23 1.215 1.26

- 109.8033 - 109.7348 - 109.7044

a9c-t h.9 0 71.0a

HIN+=NH invertomer rohmer

1.22 1.20 1.225

- 110.1360 - 110.0880 - 110.032s

30.1 64.9

- 0.483

HN=NCH3 (bans) HN=NCHo (tic) invertomer (H inverts) invertomer (CH; invert51

- 148.8032 - 148.7907

H2 N+=NCHB invertomer

rotamer ’ The barrier of isomerizhon ‘-’

21L5

- 0.468

7.8 c--t

- 148.7193

52.6 =

- 140.7297

46.1 a

- 149.1419 - 149.1030 - 149.0371

65.8

- 0.466

24.4

is given with reference to the energy of the rrans-isomer.

Energy difference between cis rmd fmns.

tion to the adopted values, we have opthrdzed the

double bond in H,C=NH, HN=NH and their various conformers. Optimization of the central bond was also performed on the protonated spectes of H,C= NH and HN=NH. The same optimized central bond

distances were employed in the calculations on the corresponding methylcompounds. The protons introduced on protonatlon were assumed to form pure sp2-hybridized N-H bonds. Optimization of the N-H bond lengths In the protonated species showed only slight variation of the total energies as a function of bond length_ The results of gross energy calculations together with the values of the optimized central bonds are given in tables 1 and 3. The barriers of isomerizitioi and the broton affinities are also included. In ad& tion to gross energy calculations, population analyses

were also made and the results are presented In tables 2 and 4 in the form of gross atomic charges and dipole moments. Comparison with experimental data can only be made In one case, for H,C=NCH3, where an experimental dipole moment of 1.53 2 0.02 D has been reported [ 151. These population analyses together with the energy calculations served as a basis for density plots, which have been performed by means of the program DENSITY [ 161. As ws pointed out by Scrocco et al. [17], electrostatic potentials around molecules may sometimes give useful Indications about certain reactivity properties. such.as proton afTinit.ies. For this reason, the electrostatic potenilals were plotted and two selected potential maps tithe plane of the molecules are *en In f@, 1 and 2 for H,C=NH and MS HN=NH. In order to obtain a gtid Wstration of the effect of, -.

.‘344

G. Menfnyi et aL. Isomerization of &-omerhine- and azwxmpo’unds

: Table 4 Total charges and diple moments of azo-compounds Dipole

Total charges on different atoms

Compound

moment H’

-

0.29

N2.

H’

HZ

(debye)

- 0.29

0.29

0.29

0.0

CiS

- 0.26

- 0.26

0.26

0.16

3.57

invertomer (k inverts)

-0.15

- 0.52

0.24

0.43

2.58

)+=N?,

3

- 0.34

0.04

H’

H3

0.43

0.41

0.41

3.08 0.33

H invertomer

-0.21

-0.19

0.43

0.43

0.54

N’

N’

C

H’

Hz.3

- 0.29

- 0.32

- 0.21

0.28

0.20

0.19

0.42

cis

- 0.25

- 0.29

- 0.32

0.25

0.20

0.21

3.18

(’ H inverts)

- 0.51

-0.19

- 0.26

0.42

0.18

0.18

2.52

-0.18

- 0.51

-0.13

0.23

0.21

0.17

2.82

N’

N2

C’

H’

H2

H3.4

Hs

- 0.32

- 0.04

-0.41

0.41

0.45

0.30

0.32

3.54

- 0.25

- 0.25

- 0.21

0.4 1

0.41

0.30

0.27

1.56

‘H pN=*N

\

H4 1 C 1 H’23

invcrtomer

(CHJ inverts)

2H U/N+=~N, ‘H

invertomer

s 2

’ H’34

protonatlon, selected density difference plots are also given for the same two molecules, figs. 3-6.

These plots geld the dtfference of electronic dens& ties between a protonated and an unprotonated molecule. Naturally, hi denslty difference plots, all bond lengths and angles must be kept the same for .*e two species.

,4. Disc-ion ‘. The mechanisms of inversion and rotation of the H2C=NH compound have been thoroughly &cussed ,I.

by Lehn and coworkers 17-9) .‘There is a rather good agreement between their own and our values for the barriers of inversion and rotation for methylene irntne. In thts work we shall rather try to tnquire tnto the reason for the drastic effect of protonation on the barriers of cis-rmrrs Isomerizatton. Generally speaking, the rotation mechanism can be understood as the result of two consecuttve processes. In the tit place, the n-bond is broken with a concomitant rotation of 90” around the C-N axis. In the rotated state a Ipseudo n-bond” will be formed, provided there is enough lone-pair character left on the nitrogen.

G. Merdnyi et aL, Isomerizrrtion ofazomethitw

345

and azo-compounds

‘..,

‘-. L

Fig. 2. The electrostatic potcntird in the plane of the rrans-

HN=NH molecule. The plotted potcnthl lcvcls range from -0.10 through 2.56 au. The incremenls between subsequent levels arc the same as in fig. 1. The value of the minimum potential is -0.1138 au corresponding to -71.4 kcd/molc.

I.0

,.a

l.0

..a

LO

1.1

1..

. 1..

Fig. 4. Density difference plot of the Hz&NH molecule in the plane perpendicular to the molecular plane and tntersccting the C=N axis. The levels hwe the time sequence as in tig. 3.

I..

1.1’

Fig. 5. Density difference plot of HN=NH in the plane of the molecule. Levels according to fig. 3. Fii. 3. The differenct in electronic density between prole nated and unprotonated HzC=NH. ‘Theplot is presenled in the plane of the molecules. Bond lengths and angles common for bc+h species are equal to the optjmized valuer for the protonated species. Solid Lines represent points where the protonated species have g larger density than the unprotonated ones. The converse i true for tied lines Finally, dotted lines mark equal densities. The inaements between the various levels are defmed by the geometrical sequence f 0.1. p, on either sides of the zao level, where n is an Lnteger starting from 0. The levels are given in atomic units.

Quahtatively, a strong n-overlap in the ground state will heighten a rotational barrier and a strong pseudo n-bond will diminish it. This state of affairs ham a strong bearing on C-N distances in the various conformerzz as can be seen in table 1. Upon rotation, the C-N distance in H,C=NH Increases by less than 0.02 k In contrast to this, the increxz of the central bond length amounts to 0.1 18 for the rotamer of the protonated H+NH. Estimated overlap changes

346

G. Merbyi

et al,. Isomerization

of azomerhine-

Fig. 6. Density difference plot of HN=NH perpendicular 10 the molecule plane intcrsecling the N=N-bond Levels PCCOIding to I’& 3.

strongly support the same picturk. On protonation, the n-bond overlap diminishes rather drastically (from 0.51 for H,C=NH ro 0.38 for H,C=N+H$. This can be related to the increased n-bond polarization as seen from the density difference charts (fig. 4). There is a transfer of n-charge towards the nitrogen during protonatlon. As regards the formation of the pseudo n-bond, tNs will occur readily for the rotated H,C=NH, since the lone pair on nitrogen can pariially compensate for the lost n-bond. TNs possibility, however, is lost for the protonated species, since the lone pair Is now effectively blocked by the proton. The pseudo %overlap is estlmated to be 0.22 for the rotated H,C=NH and only 0.1 for the rotated HzC=N+Hz. In view of the high rotational barrier of the protonated H2C=N+HZ we varied the dihedral angle between the H-C-11 and H-N-H planes in the rotated state. The calculations conflrmed that the most stable dihedral angle is 90”.

As is well known, the ground state and the first doubly excited state become nearly degenerate upon rotation by 90” round the C-N axis. Since both states have the same symmetry, the ground state energy is expected to decrease as a result of repulsive interactton with the first doubly excited state. Therefore. a Cl treatment should lower the rotattonal barrlers for both protonated and,unprotonated azomethi&compounds. However, the qualitative argument

and az*compounds

expounded above will stffl be valid. The same argument applies to the dlimides. The dilmide HN=NH mblecule has a Ngt~ barrier of InversIon compared to the imines. (Our obtained value of 52 kcallmole is in excellent agreement with 50 kcallmole calculated by Lehn [7] .) Investigation of the orbital energy changes during Inversion gives a hint why this is so. The orbital describing the lone pair is seen to be the one responsible for raising the energy. In the case of methylene imine. there is only one such orbital, which increases by 0.8 au on inversion. However, in trons-NZH2 there is an ap and a b, orbital, increasing by 0.7 and 0.6 au on inversion. Physically, this state of affairs reflects an increased lone-pairlone-pair repulsion, which partly explains the relatively high barrier of the diimide. On protonation. the inversion 52 kcal/mole

barrier of diimide diminishes from to 30 kcal/mole. The latter value

compares favourably tith the inversion barrier of H,C=NH. Comparison of the Walsh diagrams of H+NH with that of H,N+=NH yields gross similarities. Subtle differences, however, indicate why H2N+=NH has a somewhat higher barrier than H2C=NH. In H2C=NH the sum of the energies of the orbitals, essentially describing two C-H bonds and the N-H bond, descreases slightly (by 0.2 au) on inversion. In contrast, in HzN+=NH, the sum of the energies of the orbitals describing the N-H bonds increases by about the same amount on inversion. Once more, this calculation indicates that the Irunsdiimide is more stable than the c&form. Our Calculated difference of 8.9 kcal/mole agrees quite well with the value of 10.5 kcallmole as obtained by Lehn [7]. Introduction of the methyl-group does not seem to have any marked influence on the rotational barriers of imines. However, there is a lowering of the inversion barriers of both imines and diirnides by approximately 4 kcallmole. Population analysts shows, that the methyl group favours the invertomer by hyperconjugative effects. This is reflected by the changes in a-orbital energy. The a-energy of the ground state of both H,CXUH and HN=PrH increases by about 0.35 au on introduction of the methylgroup, while the Invertomen only change by some 0.2 au in n-orbital energy. The proton affinity of H$=NH is surprklngly

C.

Mere’nyi er aL.

Isomeriurtion

high There is no experimental test for this. However, azo.methlne compounds appear to be rather strong proton acceptors [6, 18, 191. But there is no direct possibility to compare pKa-values and proton affinities. For diimide we have also obtained a rather high proton-affinity. D’Mdes are known to act as bases and some of them serve In fact as acidbase-indicators. The negative potential contours on the potential chart of CH,=NH and HN=NH can be correlated to proton affinities. The potential well of the H$=NH is deeper by 22.3 kcallmole than that of the HN=NH. T,his figure is in excellent agreement with the difference in proton affinities of the two compounds, which amounts to 23.3 kcaJ/ mole. The introduction of a methyl-group results in an increased proton affinity in both cases.. This compares favourably with the experimental fact that alkylated amines are stronger bases than ammonia.

5. Conclusions The above calculations

show that the rotational

barriers of azomethinecompounds drastically increase on protonation. A protonated imine cannot invert

for obvious reasons. Therefore, the only way to isometize will be through rotation. The calculations indtcate that any reaction where cis+fmns-isomerizadon of the imine is the rate-determining step, should slow down in strongly protonated media. (Unless, of course, hydrolysis occurs in the presence of water.) No quantitative experimental data exist which support or contradict this hypothesis. The results for the azocompounds indicate that inversion barriers decrease on protonation. The mode of cis + buns-isomerization is then Likely to be inversion and the rate of isomerization should Increase on protonation. Experimental investigations seem to confm U& concluston. After the preparation of this paper, it has come to our knowledge that the problems treated in this work, have also been partly covered in a recent arttcle by Kollman and coworkers [20]. Thus the molecule

ofazomerhine-

and azo-compounds

341

methylene lmIne has been similarly treated and the results and conclusion of our and their calculations agree. Acknowledgement This work has recetved t%rancial support from The Swedish Board for Technical Development, Contract 72-98/U62, and The Swedish Natural Research Council, Contract 2741.

References [l] CC. McCarty. The chemistry of the carbon-nitrogen double bond, ch. 9. [2] D. Schulte-Frohhnde. Ann. Chem. 612 (1958) 138. [3] G. Wettermark. M.E. Lngmuir ant-JD.G. Anderson, J. Am. Chem Sot. 87 (1965) 476. 141 J.W. Smith. The chemistry of the arbon-nitrogen double bond, ch. 5. [S] S. Hunig, H.R. Muller and W. Thier, Angew. Chcm. 77 (1965) 368. [6] R-L Reeves md W.F. Smith, J. Am Chem. Sot. 85 (1963) 724. [7] J.M. Lehn and B. Munsch, Thcorct. Chim. Acta 12 (1968) 91. [8] J.M_ Lehn, 0. Munsch and Ph. Millie, Theoret. (Shim. Acta 16 (1970) 351. [9] B. L&y, Ph. hJiBie, J.M. Lehn and 8. Munxh. theoret. Chim. Acta 18 (1970) 143. lo] D.P. Wang. W.H. Fink and L.C. Allen, J. Chem. Phyr 52 (1970) 6291. 1l] K Rosengren and G.C. Pimentel, 1. Cbem. Phyr 43 (1965) 507.

121 J.B. Moffat, Can. J. Chem. 48 (1970) 1820. 131 P. Sicgbahn, private communi~tion. 141 B. Roos and P. Siegbahn, Theoret. Chim. Acta 17 (1970) 209. 1151 K.V.L.N. Sastry and R.F. Curl, J. Chcm. Phys. 41 (1964) 77. [ 161 J. Almlof, private communication [ 171 R. Boneccomi, C. Petrogolo, E. Saocco and J. Tomti. Quantum aspects of helerocyclic compounds in chemistry and bioehetilry, The Jeruulem Symposium on quanhrm chemistry. VoL 2 (1970) p. 181. ]18] P. Forslid, privnle communication 1191 G. Olah and P. KxienbuhJ, J. Am. Chem. Sot. 89 (1967) 4756. (201 P.A. KoUman et al.. I. Am Chem. Sac. 95 (1973) 21.