Trimethylamine N-oxide, structure and bonding investigated by gas electron diffraction and ab initio MO calculations

Journal of Molecular Structure, 263 (1991) 299-310 Elsevier Science Publishers B.V., Amsterdam

299

Trimethylamine N-oxide, structure and bonding investigated by gas electron diffraction and ab initio MO calculations Arne Haalandl and Hanne Thomassen Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo 3 (Norway)

Yngve Stenstrram Department of Biotechnological Sciences, Agricultural University of Norway, P.O. Box 40, N-1432 AS-NLH (Norway) (Received 29 April 1991)

Abstract The molecular structure of trimethylamine N-oxide has been determined by gas electron diffraction with a nozzle temperature of 150°C and the structure and bonding investigated by HF MO calculations at the 6-31 G*level. Structure parameters from experiment (r,, L,) and calculations (r,,L,) are: NO, r,=137.9(3), r,=137.0 pm; CN, r,=149.6(2), r,=147.3 pm; L,CNO=108.9(2)“; L e= 108.7 ‘. The uncertainties are estimated 2a values. A scaled quantum mechanical force field has been derived. The calculated energy of the reaction Me,NO+pyr+Me,N+pyr*O; AU&s = -7.1 kJ mol-‘, combined with the experimental N-O bond dissociation enthalpy of pyridine N-oxide yields a N-O dissociation enthalpy of AHzg8~295 f6 kJ mol-’ in MeaNO. Comparison with the NO bond distances and bond energy terms in H,N-OH and XN=O, X=H, Cl or F, shows that the NO bond strength in Me3N0 is intermediate between normal single and double NO bonds. Population analysis indicates that if the 0 atom in the valence state (1s)*(2s)*(2p,)*(2p,)* is taken as the point of departure, NO bond formation is accompanied by donation of 0.84 electrons into the vacant pa (2p,) orbital and back donation of 0.08 electrons from each of the two px (2p,, 2p,) orbitals.

INTRODUCTION

A normal bond in a neutral molecule has been defined as a bond which on minimum energy rupture in the gas phase yields two uncharged radical fragments [ 11, for instance Me&-F(g)+Me,C!-(g)+F*(g) Me3N0 is isoelectronic

with Me&F. Since the ionization

energy of trimethyl-

‘Author to whom correspondence should be addressed.

0022-2860/91/$03.50

0 1991 Elsevier Science Publishers B.V. All rights reserved.

300

amine is higher than the electron affinity of the 0 atom, minimum-energy rupture in the gas phase will proceed heterolytically to yield neutral diamagnetic species. Such bonds have been defined as dative [ 11. Since fluorine is a perennial rule-breaker in structural chemistry, Me,NO rather than FBNO may be regarded as the simplest, prototype species containing a dative N+O bond. The gas-phase dissociation energy of the adduct Me3NBH3, which is isoelectronic with Me,NO, is 145 ? 2 kJ mol-l [ 11, while the energy of atomization of cubic boron nitride yields a mean B-N bond energy of 385 kJ mol-l [ 11. Dative bonds to main group metals and metalloids are generally much weaker than normal (covalent) bonds between the same atom pair. When writing a review of dative bonding to main group metals, we were surprised to discover the neither the N-O bond energy in Me,NO nor the gasphase molecular structure were known. The reason is undoubtedly that it requires some care to prepare the anhydrous compound, and that the vapor pressure of pure Me,NO is low, approximately 10 Torr at 150’ C. The crystal structure of Me,NO has been determined by X-ray diffraction [ 21, but since the low vapor pressure and high melting point (225 oC ) indicate that the intermolecular interactions are particularly strong, we decided to determine the molecular structure in the gas phase and to investigate the bonding by SCF MO calculations. EXPERIMENTAL

Preparation of dry trimethylamine N-oxide Although prepared several times previously [ 31, a modification of a previously published procedure [ 3 (a) ] worked best in our hands. Commercially available trimethylamine N-oxide dihydrate from Fluka was used. All operations were carried out under an atmosphere of nitrogen. The dry Me,NO was manipulated in a glove box. Approximately 3 ml water were separated from a refluxing mixture of 9.823 g (88.4 mmol) trimethylamine N-oxide dihydrate in 50 ml benzene by means of a Dean-Stark trap. The rest of the benzene was distilled off and the solid residue was sublimed at 140-145 ‘C to give 6.245 g (94% ) of anhydrous Me,NO; m.p. 223-4°C (lit. [3(c)] 225-7°C). Electron-diffraction data The gas-phase electron diffraction data were recorded on Baltzers Eldigraph KD-G2 with a nozzle temperature of about 150’ C. Exposures were made with nozzle-to-plate distances of 50 cm and 25 cm using an accelerating voltage of about 42 kV. The plates were photometered and the data processed by standard

301

procedures [4], except that an automatic background subtraction program based on the procedure described by Hedberg [5] was used on the individual curves in a modified form. The resulting modified molecular intensities extended from s, 20.00-147.50 nm-’ with increments, ds, 1.25 nm-’ (five plates, 50 cm) and from s, 50.00-280.00 nm-’ with increment ds, 2.50 nm-’ (six plates, 25 cm). The electron wavelength was calibrated against diffraction patterns of gaseous benzene which have r,(C=C) = 139.75 pm [6] and the estimated uncertainty in the s-scale was 0.1%. The elastic atomic scattering amplitudes and their phases were calculated by using an analytical representation of the atomic potential [ 71 by the partial-wave method [ 81 for C, N and 0 and by the electron density for bonded hydrogen [9] for the H atom. The inelastic scattering factors were those of Tavard et al. [lo]. The molecular intensities were modified by multiplication with s/ ]fN 1If0 I. THEORETICAL

CALCULATIONS

Molecular structure Calculations at the ab initio Hartree-Fock SCF level with use of the program GAUSSIAN-88 [ 111 and a 6-31 G* basis set were carried out for the molecules Me3N, Me,NO, pyridine and pyridine N-oxide. The Hartree-Fock energy and the thermal energy for these molecules are given in Table 1. For Me,NO, models were restricted to those having overall symmetry C,. Six parameters were used to describe the geometry, r(C-H), r(C-N), r(N-0), L O-N-C, L N-C-H and r O-N-C-H (taken as zero for the eclipsed conformation of the C-H and N-O bond). The torsion angles were calculated to 180’ which gives a C,, symmetry. Results of this calculation are given in Table 2. Vibrational force field We needed a force field to calculate vibrational amplitude quantities. Such information was needed to perform a correction for vibrational effects in the TABLE 1 Electronic and thermal energies of the species involved in the reaction Me,NO(g) + pyridine(g) +MeaNO (g) +pyridine N-oxide(g)”

Hartree-Fock energy Sum of thermal energies

Me,N

MeeN

Cr.H,N

C,H,NO

- 173.2684642

-248.0451192

- 246.6958199

-321.4737676

0.1347418

“All quantities in Hartree per particle.

0.1414056

0.0994358

0.1046751

302 TABLE 2 Results for trimethylamine N-oxide”; molecular symmetry Csv Parameter

Ab initiob (r,,L,)

Experiment (r,,L,)

Vibrational amplitudes

r(C-H) r(N-0) r(N-C) LHCN LCNO L CNC rHCN0

1080 137.0 147.3 108.8 108.7 110.2 180.0

110.9(3) 137.9(3) 149.6(2) 107.7(2) 108.9(2) 110.0(2) 180.0

7.72 4.95 5.17

Non-bonded H.--H N.--H o*.*c c*..c O*..H, Cs. - -HI,, C 3...Hg O...H, CB.- ‘HI1 RB

distances’ 177.2 208.8 231.3 241.5 255.3 238.3 298.1 324.0 361.2

183.8(5) 210.7(3) 233.7 (5) 244.9(3) 256.1(6) 265.0(6) 266.0(6) 326.2(3) 338.0 (3) 3.07

1’oba

Id cdc

D”

(14)

7.89

-2.97

(16)

;‘;;

-0.18 -0.15

10.7

(4)

12.4 11.2

7.1 7.0

(4)

;.:

18.2 1 16.2 (5) 17.5 10.4 10.8 (6)

18.5 16.5 17.8 10.5 10.9

- 4.59 -1.40 -0.03 -0.03 0.06 -0.15 - 0.36 -0.96 -0.82

“Distances (r) and amplitudes (1) in picometers, angles ( L ) in degrees. Uncertainties in parentheses are estimated 2a values; values for r, are as for r,. bCalculated with the 6-31 G* basis. ‘Quantities in braces were refined in groups. dCalculated from the scaled ab initio force field. “D=l’/r-K-dR. fH - * *H distances over more than one bond angle are omitted from the table; l-values for all H* - *H distances were kept at the calculated values. I’* where di=Zi(obs)-Zi(calc). ‘R=lOO[~wid:/CwiZ:(obs)]

structure determination (shrinkage corrections) and to establish reasonable estimates and constraints for vibrational amplitude parameters (l-values). Scaled quantum mechanical (SQM) force fields are considered as approaching the best accuracy which can be achieved in the harmonic treatment [ 121. We calculated the quadratic (Cartesian) force field of Me,NO for the optimized structure at the ab initio HF/6-31 G* level with use of the analytic second derivative package of GAUSSIAN-88. The Cartesian force field was converted to one defined in terms of the symmetrized internal coordinates in Table 3. A set of scale factors for the symmetrized internal force constants was then determined by fitting wavenumbers calculated by GAUSSIAN-88 to those observed by Choplin and Kaufmann [13]. This scaled quantum mechanical

303

(SQM) force field is given in Table 4. Wavenumbers for the fundamentals found in Table 5.

are

STRUCTURE ANALYSES

Model

As for the theoretical calculation, models were restricted to those having overall symmetry C, and six parameters were used to describe the geometry, TABLE 3 Definition of symmetry coordinates and scale factors for trimethylamine N-oxide Scale factor

Symmetry coordinate”

=1

0.850 0.840 0.830 0.820 0.890 0.870

Sl S2 S3 S4 S5 S6 A (a736+ + au,,,+

a638+

a637 + &0,4,11+

a,,,+

a,,,+

~1.4,10+

a94,11+ a1,4,11+

a94,10+ %5,12

+*2cf149~1,4,10a1,4,11+ 2%5,12S7 A(2a136-%7-a136 S6 A~~~736-~636-~637~~~lo,4,~~-~94,1l-~94.l0+2~l3,5,~4-~lZ,5,l4-~l2,5,l3)

o”T745 0.700 0.720 1.ooo

S9 A(r37-~3~+r410-~411+r~13-r514)

E.770 0.790 0.820 0.745 0.800 0.830 0.920

slo

A (al37

sll

A(@63,-

a637 + ~,,,ll-

&2

A(71236+

71237+ 71236+ 51249+ 7124,10+ %!4,11+~125.12+

fl1,4,10-

%,4,11+

a1.5.13 -

&94,10 + a12,5,14 -

%.5,13 -

a1.5.14 )

a~5.14) (y12,5.13 ) 7125,13+ 7125.14)b

f%3A(2~13-~14-~15) ~14A~2~36+~~37+~~36-~49-~410-~411-~512-~513-~514) sl5

A(4~36-2~37-2~36-2~49+r4~o+~411-2~512+~513+~514)

S16AC sl7

)

r410-r411+r5~3-r514

A@a213-a214-a215)

s16 A W,l,-aa,,-

a314) ~1o,4,11-~94.11-~9,4,lO-~l3,5,14-~12,5.14-(y12,5,13

sl,A(%36+2~636+2~637-2a

0.830 0.700 0.720 0.720 1.000

%36+

12,5,13+ al36

+ a1.5.13 + %5,14)

0.760 0.725

-

a12,5.14 +(Y

&3,5,14+

136-%37-%36+

~149+~1,4,10+%4,11+ %49+

s20A(4~136-%37-2@-136S21 A(a

2~10,4,11+

522 A @a736

-

2a636 -

S23 A(~194,u

-

~94,lo+%,5,14-%2,5.13)

s24 Awl236+

271237 + 27 x236-

al,5,13+al,5,14

2~l.5,~2+%,5,13+%5,14

)

)

)

1,4,10-%4,11-~1.5,13+%5.14 2a637-

%4,11+al,5~12+

~1,4,10+~1.4,11-

71249-

a94,11+

7124,10-

a9,4,1o -2&13,5.14+

7124,11-

h25,12-

a12.5,14 +a

%25,13-

12.5.13)

7125,14)b

*Atom numbering from Fig. 1. Normalization factors have been omitted. “The torsion ( 7i,kl) coordinates are defined according to ref. 26. Tijklis positive when, viewed in the direction rZ+j, r,, rotates counterclockwise with respect to rib

304 TABLE 4 Values of quadratic SQM HF/6-31 G* force constants for trimethylamine N-oxide”

a,

al

S9 S, S2 S3 S4 S5 SL? S7 54

4.387

4.697 0.609

- 0.007 0.057 0.709 0.028 0.196 -0.002

4.918 0.114 0.031 -0.318 -0.371 - 0.045 -0.017

5.051 -0.017 -0.022 0.367 0.034 -0.015

4.894 0.656 -0.014 - 0.339 - 0.094

e S 13 S 14 S 15 S 16 S 17 S 18 S 19 S 20 S 21 S 22 S 23 S 24

1.404 0.055 0.051 0.094 S 24

4.037 0.156 0.029 0.002 0.200 -0.348 - 0.348 -0.014 -0.051 -0.015 - 0.003 - 0.002

0.050 4.772 -0.030 0.005 -0.012 - 0.005 0.381 0.048 -0.013 -0.015 0.001 -0.001

4.814 0.006 0.377 0.087 -0.012 -0.327 - 0.008 -0.118 -0.005 0.023

4.465 - 0.090 0.502 - 0.002 - 0.003 - 0.347 0.002 - 0.099 0.016

1.198 -0.195 - 0.004 0.097 0.041 0.033 - 0.035 0.058

S10

S,,

S12

-0.131 0.524

-0.325 - 0.089 0.662

0.040 -0.001 0.021 0.029

0.614 -0.012 0.011

0.778 - 0.074

s

S22

23

-0.011 0.562

1.305 0.055 0.040 0.011 0.025 0.098 -0.053

0.002 0.001 0.543

0.620 -0.019 0.005 0.009 0.002 -0.000

0.568

f&l 0.026 -0.103 0.005 0.796

0.796 0.021 - 0.069 - 0.004 - 0.004

“Ab initio values scaled with factors from Table 2. Units are consistent with energy in attojoules and coordinates in angstroms or radians.

r(C-H), (r(N-X))=3/4r(C-N)+1/4r(N-O),d=r(C-N)-r(N-O), LON-C, L N-C-H and z O-N-C-H. The structure was defined in terms of the spatially consistent ra type which were converted to the r, type required by the electron-diffraction scattered intensity formula according to r, = r, + 6r + K- P/r. Anharmonicity coefficients (K) were calculated from ~=a,Z~/6. The Morse anharmonicity parameters were given the values 19.8 nm-’ for C-H, 25.5 nm-’ for N-O and 22.7 nm-’ for C-N [ 141. The anharmonicities for all non-bonded distances were ignored. Values of 6r, K and 1 in the distance conversion and in the calculations of K were given the values calculated from the SQM force field using the program ASYMBO [ 151. Refinement results It was not possible to refine the methyl torsion angle, but the lowest Rfactors were found for torsional angles of 180’ which give a Csvsymmetry. We

305 TABLE 5 Wavenumbers (cm-‘) Fundamental mode

a1

a2

e

for fundamentals of trimethylamine-N-oxide &bs

PEDb

wale

0.89”

SQMd

1 2 3 4 5 6 7 8

3030 2948 1480 1395 1255 935 765 466

2973

2901 1485 1445 1243 908 749 429

3030 2949 1479 1390 1252 932 771 460

S,@8) S,(98)

S,(lOl) SdlO7) S,(73), S,(81), S,(75), &(95),

9

-

2980

2896

S,W)

10 11 12

-

1440 1037 217

1421 1002 225

S,,(lO3) S,,(lOl) &,(103)

13 14 15 16 17 18 19 20 21 22 23 24

3020 2900 2870 1456 1445 1389 1241 1124 945 490 364 -

2987

3021 2901 2866 1459 1450 1389 1253 1121 941 484 378 294

S15W) S,,@8) S,,W) S,,@9) &!, (97)

2962

2888 1463 1452 1403 1278 1105 958 476 366 285

G(l9) %(20) S,(13) S,(l4)

s,,(1091

&,(35), Sm(41) &?,(75L &(17) &,(58), f&,(35) &,(43), &3(34) S,,(57), &8(55) h(91)

“Ref. 13. bSymmetry coordinates indicated are those dominant in each normal mode. For definition of symmetry coordinates see Table 3. “Ab initio values multiplied by factor. dSQM values calculated from the SQM force field of Table 4.

also tried to refine differences in the angles L NCHG and L NCH, (see Fig. 1). Such refinement, however, gave large uncertainties in the determination of these two angles, L NCH 6= 106.3 (18) ’ and L NCH, = 108.4 (10) “; significant correlation between them, p= - 98; no changes in the other parameters and no significant improvement in the R-factors. In our final model these two angles were kept equal. Some of the vibrational amplitudes were combined in groups and handled as a single parameter with intragroup differences held at the calculated values. Z-values for He * *H distances were kept at the calculated values as they give relatively little contribution to the intensity. The groups are evi-

306

b---

0

-

50

100

150

200

250 s,

300

nm(-1)

Fig. 1. Observed modified molecular intensities ( l ) for trimethylamine N-oxide and calculated counterparts (full line) from the model in Table 2. The corresponding difference intensity curve is shown below.

TABLE 6 Least-squares standard deviations (0,s) and correlation matrix (pij) for parameters of trimethylamine-N-oxide” Parameter

r(C-H) (r(N-X))

A LCNO LHCN

LC-H 10-N 1N.e.H

lO**.C 10**.H6 10*..H7 Sib S2b

fJLS

1OOpij

0.65 0.26

100 30 0.97 -14 0.10 -16

99 -35 -47

0.11 0.70 0.79 1.9 1.8 2.8 2.4 0.03 0.06

-11 3 36 100 26 -16 -12 -6 100 20 -64 -22 -7 33 0 140 6 0 -27 9 83 20 1 -11 2 15 3-2 20 -7 33 -9 11 2 -5 5 -10 18 0 0 5 -5 33

-20 -1 -14 0 -15 -3 -1 -17 -16

100 42

100

100 8 11 3 18 45 51

100 64 3 46 19 16

100 10 100 60 -6 100 30 9 27 100 26 7 15 44 100

“Distances (r) and amplitudes in picometers, angles in degrees. For numbering of atoms see Fig. 2. For explanations about grouping of the amplitudes see Table 2. bThe scale factors were Sl,10.02 and S2,10.01.

307

i

0

100

200

300

500

400

r. pm Fig. 2. Experimental ( l ) and theoretical (full line ) radial distribution curves and the corresponding difference curve for trimethylamine N-oxide. The damping coefficient is 1.2 x 10m5 nm*. The distance distribution is indicated by vertical bars. The numbering of the atoms is also given.

dent from Table 2, which contains the final structural results. Table 6 is the correlation matrix of the final model. Calculated intensity curves and radial distributions appear in Figs. 1 and 2. DISCUSSION

Both the ab initio MO calculations and the gas electron diffraction data indicate that the equilibrium conformation of Me,NO has C,, symmetry as indicated in Fig. 2. The experimental and calculated structure parameters are listed in Table 2. The agreement is satisfactory considering that bond distances are known to be calculated 2-3 pm too short at the Hartree-Fock level. Comparison with the X-ray study of Caron et al. [2], r(C-H) = 101(7) pm, r(NO)=138.8(5) pm, r(NC)=147.7(6) pm, LCNO=110.6(6)” LCNC= 109.0 (6) ‘, gives no indication of any significant differences between the gaseous and crystalline states. Calculations of electronic and thermal energies of the species involved in the reaction Me,NO(g)

+pyridine(g)+Me,N(g)

+pyridine N-oxide(g)

yielded the energy of the reaction AiLK&= - 7.1 kJ mol-l.

We assumed the

TABLE 7 Orbital populations, gross atomic population and atomic charges with the 6-31G* basis set for trimethylamine N-oxide and trimethylamine Me,NO 0 Orbital population S 3.96 PX,PY 1.92 P* 0.84 Id 0.03 Atomic charges 0 - 0.67 N -0.20 C - 0.28 +0.19 (H)

Me3N N 3.52 1.21 1.15 0.11

N 3.59 1.08 1.73 0.05

-0.53 -0.29 +0.15

error in this quantity to be less than 5 kJ mol-l. Combination with the experimental dissociation enthalpy of pyridine N-oxide, 301.7 + 2.8 kJ mol-l [ 161, yields an estimate of the dissociation enthalpy of the NO bond in Me3N0 of AH&s =295&6 kJ mol-l. Comparison of the NO bond distance in Me,NO with the single bond distance in H,N-OH, r,= 145.3 (2) pm [ 171 and with the double bond distance in HN=O, r, = 123.9 (5) pm [ 181, shows that the dative N-0 bond in Me,NO is intermediate between normal single and double bond NO distances. Similarly, comparison of the N-O dissociation enthalpy with the bond energy terms in H2N-OH and ClN=O, 175 and 587 kJ mol-’ [ 191, respectively, indicates that the strength of the dative NO bond in Me3N0 is intermediate between the strength of normal single and double bonds. The dipole moment of Me,NO in benzene is 5.03 D [20] compared to 0.60 D for gaseous Me,N [ 211. The difference suggests that about 0.7 e- has been transferred from N to 0. The calculated dipole moment is 4.96 D in good agreement with the experimental value. Taking an 0 atom in a (1s)’ (2s)’ (2p,)‘( 2~~)’ valence state as our point of departure, Mulliken orbital populations (Table 7) suggest that formation of the dative bond is accompanied by donation of 0.84 electrons into the empty po (p,) orbital on 0 followed by back-donation of 0.08 electrons from each px (p, or p,) orbital. The elongation of the N-C bonds on going from Me,N to Me,NO, 145.4 (2 ) pm [ 221 to 149.6 (2) pm may reflect repulsion between 0 and C atoms. The great strength of the dative bond in Me,NO compared to Me3NBH3, dissociation enthalpies of 295 + 6 kJ mol-l vs. 145 + 2 kJ mol-’ [ 11, probably

309

reflects both the greater electronegativity of 0 compared to B and the possibility of back-donation of lone pairs on the acceptor atom. The NO bond distance in F3N0 is 115(4) pm [23], about 20 pm shorter than in Me,NO. The calculations of Scheyer and co-workers [ 241 suggest that in this compound bond formation is accompanied by donation of a full electron into the pa orbital on 0 followed by back donation of 0.26 electrons from each pn into dr N-F orbitals. The N-F bonds in F,NO, 143.1(3) pm, are elongated by nearly 7 pm relative to this bond in F3N, 136.48 (20) pm [ 251. ACKNOWLEDGMENTS

We are grateful to Ing. Snefrid Gundersen for densitometry measurements on the photographic plates and to Ing. Hans V. Volden for recording the electron diffraction diagrams. Y.S. is grateful to Statoil under the VISTA program administered by the Norwegian Academy of Science and Letters for financial support.

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