The molecular structure of ortho- and meta-fluorobenzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

J o u r n a l of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 443 (1998) 9-16

The molecular structure of ortho- and meta-fluorobenzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations T . G . S t r a n d a, M . A . T a f i p o l s k y h'*, L . V . V i l k o v b, H . V . V o l d e n a aDepartment of Chemist~, University of Oslo, Box 1033 Blindern, N-0315 Oslo, Norway bDepartrnent of Chemistry, Moscow State University, Moscow 119899, Russia

Received 28 July 1997; accepted 1 September 1997

Abstract

The molecular structures of gaseous o-fluorobenzaldehyde and m-fluorobenzaldehyde have been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations (at HF/6-311G** level). The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters (rg) from the joint ana~sis with estimated total errors (in parentheses) are for o-fluorobenzaldehyde: (C-C) . . . . = 1.399(2) A, C - F = 1.334(5) A, C-C(=O) = 1.515(6) ,~, C=O = 1.216(3) ,~, /-CCvC = 122.0(2) °, Z-CCcHoC = 120.3(6) ° and for m-fluorobenzaldehyde: (C-C) . . . . = 1.394(2) A, C - F = 1.346(4) A, C-C(=O) = 1.494(4) ~,, C=O = 1.201(2) ~,, Z_CCvC = 122.3(1) °, and Z-CCcHoC = 120.6(3) °. The scaled molecular force fields have been determined. © 1998 Elsevier Science B.V. Keywords: Benzene derivatives; Electron diffraction; Ab initio calculations

1. Introduction

Recently we have studied the molecular structure o f p-fluorobenzaldehyde in the gas phase [1]. A shortening o f the C - F bond distance was found there in comparison with that obtained previously for fluorobenzene. The present work is devoted to an investigation o f the mutual influence through the benzene ring between a formyl group and fluorine atom in ortho and meta positions, o- and m-fluorobenzaldehyde have been studied by microwave spectroscopy [2]. In order to reproduce experimental rotational constants the most part o f structural parameters were assumed * To whom correspondence should be addressed.

there. Only the O-trans form o f o-fluorobenzaldehyde was detected in the gas phase (see Fig. 1). From relative intensity measurements for m-fluorobenzaldehyde the O-cis form was shown to be more stable than the O-trans form by 0.3(2) kcal mol -l. In this work we attempted to obtain molecular structures o f o- and m-fluorobenzaldehyde by joint analysis o f electron diffraction and microwave data with constraints from ab initio calculations.

2. A b initio calculations

The numbering o f atoms is shown in Fig. 1. A b initio calculations were carried out with the

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0022-2860(97)00365-7

10

T. G. Strand et al./Journal of Molecular Structure 443 (1998) 9-16

H14

O13 ~

~O13

~

H14 C12

C12

Hll

F7 C6

C2

(a) H10

HIO H9

H9

O-cis

H14

O-trans

O13

~.013 C12

H14 C12

HI1

H7

Hll C

C6

C6

(b) F8

NlO

H9

O-cis

H10

F8 H9

O-trans

Fig. 1. Numbering of atoms: (a) o-fluorobenzaldehyde; (b) m-fluorobenzaldehyde.

Gaussian 94 program package [3] using the 6-31G** and 6-311G** basis sets at the Hartree-Fock level. A full geometry optimization constrained to C~ symmetry has been done. In order to determine the barrier to internal rotation with respect to the formyl group rotation, a scan of the potential energy surface has been done in steps of 30° in the range 0-180 ° for the/_C 2C iC 12013 dihedral angle using HF/6-31G**. The geometry has been optimized for each step assuming a planar benzene ring and formyl group. There are two minima in the energy surface for both o-fluorobenzaldehyde and m-fluorobenzaldehyde. The O-trans form is more stable than the O-cis form by 3.2kcalmol -~ in o-fluorobenzaldehyde. The O-cis form is more stable than the O-trans form by 0.1 kcal mol L in m-fluorobenzaldehyde. The computed bond distances and valence angles of the most stable forms are listed in Table 1, Table 2. The molecular force fields

of o- and m-fluorobenzaldehyde were calculated at the HF/6-311G** level of theory and used in a normal coordinate analysis.

3. Normal coordinate analysis The definition of the internal and symmetry coordinates are given in Ref. [1] (see Fig. 2 and Table 2). The ab initio force constants were scaled as suggested by Pulay et al. [4] in order to reproduce the experimental set of frequencies [5]. The scale factors used were the same as in Ref. [1] (see Table 3). The rootmean-square amplitudes of vibration (u), the perpendicular amplitude correction coefficients (K), and the set of harmonic corrections to the ground state rotational constants (6Bvib) were calculated from the scaled force field at experimental temperatures. For

T.G. Strand et al./Journal o f Molecular Structure 443 (1998) 9-16

11

Table 1 Structural parameters of o-fluorobenzaldehyde (distances in ,~, angles in degrees)

Table 2 Structural parameters of m-ftuorobenzaldehyde (distances in ,~, angles in degrees)

Parameter

GED (rg, /_ ~,)

GED + MW

HF/6-311G** (re)

Parameter

GED (rg, /_~)

GED + MW (ro)

HF/6-311G**

C ~-C 2 C2-C3 C3-C4 C4-C5 C5-C6 C 6-C ~ C 1-C 12 C F C 12-O 13 (C-H) ....

1.397(2) 1.393(2) 1.397(2) 1.404(2) 1.393(2) 1.408(2) 1.523(5) 1.344(5) 1.221 (2) 1.103(6) 1.120(6)

1.397(2) 1.393(2) 1.397(2) 1.404(2) 1.393(2) 1.408(2) 1.515(6) 1.334(5) 1.216(3) 1.090(6) 1.108(6)

1.3823 1.3779 1.3819 1.3894 1.3784 1.393 1.4864 1.3262 1.1852 1.074 1.0908

C i-C 2 C2-C3 C3-C4 C4-C~ C5-C6 C6-C~ C i-C 12 C-F C 12-O 13 (C-H) .... C i2-H 14

1.402(2) 1.382(2) 1.394(2) 1.393(2) 1.398(2) 1.396(2) 1.496(4) 1.347(4) 1.202(3) 1.106(5) 1.128(5)

1.402(2) 1.382(2) 1.394(2) 1.393(2) 1.398(2) 1.396(2) 1.494(4) 1.346(4) 1.201 (2) 1.103(4) 1.123(4)

1.3905 1.3709 1.3821 1.3813 1.3867 1.3847 1.4881 1.3251 1.183 1.074 1.096

122.0(2) 118.0(2) 120.2(2 ) 122.0(6) 117.5(7) 120.3(6) 121.9(6) 122.8(6)

122.46 118.42 120.73 119.59 120.81 117.98 121.62 123.22

Z-CIC2C3 /-C2C3C4

/-C2C tC 12 Z_CIC i.,O13

122.2(2) 118.2(2) 120.5(2) 120.8(4) 119.2(6) 119.1(5) 123.9(6) 121.6(7)

Z.C2CIC~2 £CiCi20~3

118.3(1) 122.3(1) 119.0(1) 120.0(3) 119.9(4) 120.6(3) 118.9(9) 126.0(4)

118.3(1) 122.3(1) 119.0(1) 120.0(3) 119.9(4) 120.6(3) 118.9(3) 126.0(4)

118.36 122.22 118.94 120.07 119.89 120.52 119.72 124.35

AV (kcal mol i)

1.1(2)

1.2(2)

3.2

R factors (%) 50 cm 25 cm

2.4 4.9

2.7 4.8

CI2-HI4

/__C IC 2C 3 ~_C2C3C 4

Z_C 3C4C5 ~_C4C5C 6

/-C 5C6C i ~-C6C IC 2

R factors (%) 50 cm 25 cm

3.6 4.9

~.C3C4C 5 ~.C4C 5C 6

/-C5C6C ~ ~-C6C IC 2

4.4 5.5

a Estimated total errors in parentheses: a = [2(O'LS) 2+ (see text).

a Estimated total errors in parentheses: o = [2(aLS)2 + (0.00 lr) 2] I/2 (see text). (0.001

r)2] t/2

each torsional d e p e n d e n t distance w e used interpolated values o f the vibrational amplitudes, u(~b), w h i c h w e r e calculated for ~b = 0, 30, 60, 90, 120, 150 and 180 ° using the A S Y M 4 0 p r o g r a m [6]. The contribution from the f o r m y l group torsion m o d e was e x c l u d e d during calculations o f vibrational amplitudes and p e r p e n d i c u l a r a m p l i t u d e correction coefficients ( " f r a m e w o r k " a p p r o x i m a t i o n ) for the large a m p l i t u d e model.

inlet system w i t h the n o z z l e temperature o f about 22°C. E x p o s u r e s w e r e m a d e at n o z z l e - t o photographic plate distances o f 498.81 m m (six plates) and 248.81 m m (six plates), respectively. The electron w a v e l e n g t h was 0.058625 ,~,. All plates w e r e scanned on an A g f a Arcus II scanner and the data p r o c e s s e d with a p r o g r a m system written by T,G. Strand [9]. A t o m i c scattering factors w e r e taken from Ref. [ 10].

5. Structure refinement 4. Gas electron diffraction o - F l u o r o b e n z a l d e h y d e and m - f l u o r o b e n z a l d e h y d e w e r e obtained from the A l d r i c h C h e m i c a l Co. Gas electron diffraction data w e r e r e c o r d e d with a Balzers Eldigraph K D G - 2 unit [7,8] with a c o n v e n t i o n a l metal

Structure refinements w e r e carried out with the prog r a m K C E D 2 6 [ 11 ]. O w i n g to v e r y large correlations b e t w e e n s o m e o f the g e o m e t r i c a l parameters w e had to m a k e constraints from ab initio, H F / 6 - 3 1 1 G * * , e v e n i f the rotational constants w e r e incorporated into the analysis. The f o l l o w i n g constraints and

12

T. G. Strand et al./Journal o[' Molecular Structure 443 (1998) 9 - 1 6

Table 3 Some vibrational amplitudes of o- and m-fluorobenzaldehyde from the joint analysis of electron diffraction and microwave data (in ,~.) o-Fluorobenzaldehyde

m-Fluorobenzaldehyde

Type

rg

U~xp

u~al~

( C - C ) mean

1.399 1.334 1.216 1.515 1.090 1.108

0.046(4) 0.042(4) 0.037(4) 0.046(4)

2.44 2.39 2.40 2.43 2.43 2.44 2.35 2.35 2.40 2.79 2.76 2.84 3.60 3.64 4.09 2.54 2.50 3.83 3.77 4.30

0.065(3) 0.065(3 0.065(3 0.066(3 O.065(3 0.065(3 0.068(3 0.068(3 0.067(3 0.066(9 0.066(9) 0.067(9)

C F C=O C L-C t2 (C-H)mean Ci2 Hi4 Ct'--C3 C2...C 4 C t" .C 5 C3-"C5 C2-' "C6 C4""C 6 CI'"F7 C3""F7 CI'"OL3 CI'"C4 C2""C5 C3'"C6 C4""F7 C6"" 'F7 C5" "'F7 C2""C12

C6'"CI2 C3'"CIz C5""CI2 C4""Ct2

Type

rg

Uexp

Ucalc

0.047 0.043 0.038 0.048 0.077 0.079

1.394 1.346 1.201 1.494 1.103 1.123

0.046(2) 0.041(2) 0.038(2) 0.046(2)

0.047 0.042 0.038 0.046 0.077 0.080

0.054 0.055 0.055 0.055 0.055 0.055 0.057 0.058 0.056 0.061 0.061 0.062 0.059 0.059 0.062 0.062 0.063 0.063 0.064 0.067

2,39 2,43 2,42 2,40 2.43 2.42 2.35 2.36 2.40 2.79 2.80 2.76 3.62 3.62 4.11 2.49 2.51 3.75 3.79 4.28

0.061 (2) 0.061 (2) 0.061 (2) 0.061(2) 0.062(2) 0.062(2) 0.064(2) 0.065(2) 0.063(2) 0.064(4) 0.065(4) 0.064(4)

0.054 0.055 0.055 0.055 0.055 0.055 0.058 0.058 0.057 0.061 0.062 0.061 0.059 0.059 0.062 0.063 0.064 0.063 0.064 0.067

C 2""F 8 C4""F~

C l"'Fs Cs'"Fs C6""F8

a The braces indicate the group variation of amplitudes. b Calculated from the scaled force field (see text).

assumptions are made: 1. Planar benzene ring and aldehyde group for all values of ~b. 2. The C - C bond distances in the ring are all different, and their differences are fixed to the ab initio calculated values. Thus only one C - C distance was refined. 3. The differences between the valence angles /-CIC2C3, /-C2C3C4 and the /-C3C4C5 angle had to be constrained to the values calculated by ab initio. 4. All the C - H bond distances in the ring are set equal. The difference between the average C - H bond distances in the ring and the C - H bond distanee in the formyl group is fixed to the value calculated by ab initio.

5. All the /_CCH valence angles were fixed to their values calculated by ab initio. A joint structural analysis based upon the electron diffraction data and the rotational constants for the molecules was undertaken. The effective rotational constants for the vibrational ground state (B0) were transformed into zero-point rotational constants (Bz) using the harmonic corrections calculated from the scaled force field and the appropriate corrections to r~0 were calculated using the formula: 0 _ ra = u 2 / r a _ 1 . 5 a 3 ( u 2 _ u 2) _ Ko rc~

where uo is the root-mean-square amplitude of vibration, and Ko is the perpendicular amplitude correction at 0 K, respectively, and a 3 is the cubic anharmonicity constant which is estimated to be 2.0 ,~-~

~ G. Strand et al./Journal of Molecular Structure 443 (1998) 9-16

for C-C, C-F, and C - O bond distances, and 2.6 •-1 for C - H bond distances. The asymmetry parameters, K, for bonded atomic pairs were estimated from the diatomic approximation by K= (1/6)a3 u4. The K values for all the non-bonded distances were ignored. Correction for shrinkage was incorporated in the analysis by refining a geometrically consistent r~ structure: rc~ = ra + D

13

The potential energy function for m-fluorobenzaldehyde was taken as: v(4,) = ½v2(1 - cos

where V2 is the barrier height of the internal rotational.

6. Results and discussion

where D=ue /r-K

For each distance, depending on torsional angle (~b), we used the values of D(~b) and u(~b) which were calculated by interpolation of D and u values previously computed for ~ = 0, 30, 60, 90, 120, 150 and 180° using the ASYM40 program [6]. The uncertainties in the B= rotational constants have been estimated to be 10% of the vibrational correction ~ B v i b = B z - B o. Centrifugal distortions and corrections arising from electronic contributions were neglected. The weighting of the data originating from the two methods is chosen such that the standard deviations of the calculated quantities (Ba°) are within the limits of the corresponding estimated uncertainties in the rotational constants (0.1tSBvib). The large amplitude motion due to torsion of the formyl group was taken into account by using a mixture of pseudo-conformers with torsional angle (4~) ranging from 0 to 180° in steps of 12°. The mole fraction of each pseudo-conformer was assumed proportional to the Boltzmann factor exp[-V(4~)/RT]. The potential energy V(~) for o-fluorobenzaldehyde was assumed to have the form: V(~b)=AV+ Vl cos 0+//2 cos 2 ~ + V3 cos 3~ where AV is the energy difference between the O-cis (4) = 0 °) and O-trans (4) = 180°) forms. This function can give a barrier at 90 ° and two minima at 0 ° and 180° if Vx > 0, V3 > 0 and /I2 < 0. Taking the above conditions into account and assuming V(180 °) = 0 one can obtain for the coefficients VI, V2, and V3: V~ = 3AV

v:=-½av V3: AV

Least-squares refinements of eight geometrical parameters: C~-C2, CI-Ct2, C12-O13, C - F , the mean C - H bond distances, valence angles /-CCFC, /C2C1C12, ~CIC12013 with the constraints mentioned above converged to the values listed in Tables 1 and 2. The vibrational amplitudes are given in Table 3. The refinements were carried out using a diagonal weight matrix. The estimated standard deviations have been multiplied by x/~ to refect uncertainties due to data correlation, and further expanded to include an estimated scale uncertainty of 0.1%. Only a few correlation coeffÉcients exceed the value of 0.70 (for o-fiuorobenzaldehyde: r(C-F)/r(CE-C2) = - 0.80, r(C-F)/u(C r C 2 ) = 0.76, r ( C i - C 2 ) / u ( C t - C 2 ) = - 0.73, and for m-fluorobenzaldehyde: r(C-F)/ r(CI-C2) = - 0.81, ~.C2CIC12/~_CCO : - 0.86 and r(C-F)/u(CI-C2) = - 0.77). It should be noted that inclusion of the rotational constants in the analysis did not allow to decrease the correlation between parameters. But at the same time one can see the decreasing of standard deviations for some parameters refined in the joint analysis (see columns 2 and 3 in Tables 1 and 2), Comparing the final structural parameters we can see that the microwave data are consistent with the electron diffraction data. It means that the systematic errors are absent. The ab initio calculated bond lengths are systematically shorter than those obtained experimentally. For o-fluorobenzaldehyde the joint analysis results in the small decreasing of C IzCj2, C - F and C=O bond distances of about 0.01 A. The energy difference between the O-cis and O-trans forms in o-fluorobenzaldehyde (AV) determined experimentally is less than that obtained by ab initio calculations (see Table 1). Experimental and calculated molecular intensity curves are compared in Figs. 2, and 4. Experimental

14

T.G. Strand et al./Journal qf Molecular Structure 443 (1998) 9-16

s.(s)

~

J'~

d

,,../

A~, /', v

-v

A ------

t ,.--

f

' ;

....

,'o

. . . .

,;

. . . .

z'o

z;

. . . .

(g-t)

s

Fig. 2. Experimental (dots), calculated (solid) and difference molecular intensity curves for o-fluorobenzaldehyde.

and calculated radial distribution curves are compared in Figs. 3, and 5. The rotational constants are listed in Table 4. The agreement between the values of rotational constants derived from electron diffraction itself and those from microwave

spectroscopy (see columns 2 and 4 in Table 4) is fairly good (one exception is for the A-type constant for o-fluorobenzaldehyde). Table 5 shows some structural parameters for fluorine-substituted benzene derivatives. The C - F

f(r)

=?

¢DtD 4 1

p~

A 0

1

2

3

4

5

6

r (~) Fig. J. Experimental (dots), calculated (solid) and difference radial distribution curves for o-fluorobenzaldehyde.

T.G. Strand et al./Journal of Molecular Structure 443 (1998) 9-16

sM(s)

/• q/w-v" /• ./ w -

' ' ' ~

,.A

A

,,A

A,,

....

V

i'o ....

A

f5 ....

15

A ..--~ ,---- v

do ....

d5 ....

do

s (A-t) Fig. 4. Expcrimental (dots), calculated (solid) and difference molecular intcnsity curves for m-fluorobenzaldehyde.

bond distances found for o-fluorobenzaldehyde and for p-fluorobenzaldehyde have a tendency to be shorter than those obtained for m-fluorobenzaidehyde and for fluorobenzene itself (see Table 5). This result agrees with the concept of conjugation through the

benzene ring. The angular distortion of the benzene ring mainly affects the internal angle at the ipso carbon atom, which, as expected for an electronegative substituents (such as NO 2 group and halogen atoms) is larger than 120 ° (in benzene itself).

t(r)

A I ~'-I r~ll r~

0

:

:

:

r~ .

2

r~ .

4

ra, .

.

0

0

.

6

r (A) Fig. 5. Experimental (dots), calculated (solid) and difference radial distribution curvcs for m-fluorobenzaldehyde.

16

T. G. Strand et al./Journal (ff Molecular Structure 443 (1998) 9-16

Table 4 Rotational constants (MHz) Type

ED "

ED + MW

MW b

MW c

o-Fluorobenzaldehyde (O-trans) A 2537(9) B 1560(6) C 966(2)

2568.3(2) d 1560.91(5) 970.87(2)

2567.609(3) 1560.8694(9) 970.954(1)

2568.26 1560.83 970.88

m-Fluorobenzaldehyde (O-c~) A 2919(20) B 1268(8) C 884(2)

2920.6(3) 1269.60(10) 884.92(5)

2919.255(2) 1269.697(2) 884.964(2)

2920.58 1269.56 884.92

Calculated from the geometry above (see column 2 in Tables 1 and 2). b Ref. [2]. ~The same as in Ref. [2] but with harmonic corrections: 6Bvib= (1/2) Z ~i ~ Bz - B0 (see text). d2OLS in parentheses. Table 5 Comparison of some structure parameters (rg) in benzene derivatives Molecule

(C C) .......

C F

C C(-O)

C=O

/-CCvC

/-CCcHoC

Reference

C6HsF o-FC6H4CHO m-FC6H4CHO p-FC6H4CHO

1.396(3) 1.399(2) 1.394(2) 1.397(1)

1.356(4) 1.334(5) 1.346(4) 1.342(7)

1.515(6) 1.494(4) 1.499(6)

1.216(3) 1.201(2) 1.207(5)

123.4(2) 122.0(2) 122.3(1) 122.2(2)

120.3(6) 120.6(3) 121.4(7)

[12,13] This work This work [1]

a Distances in/k, angles in degrees.

Acknowledgements W e are grateful to the N o r w e g i a n A c a d e m y o f S c i e n c e and Letters, to the R u s s i a n F o u n d a t i o n for Basic R e s e a r c h e s ( P r o j e c t N o / 9 6 - 0 3 - 3 2 6 6 0 a ) for financial s u p p o r t a n d to the N o r w e g i a n R e s e a r c h C o u n c i l ( P r o g r a m m e for S u p e r c o m p u t i n g ) for a g e n erous g r a n t o f c o m p u t i n g time. T h e authors w i s h to t h a n k Prof. S v e i n S a m d a l for fruitful d i s c u s s i o n s o f this w o r k and to Mrs. S n e f r i d G u n d e r s e n for t r a c i n g the p h o t o g r a p h i c plates.

[4]

[5] [6] [7] [8]

References [1] S. Samdal, T.G. Strand, M.A. Tafipolsky, L.V. Vilkov, M.V. Popik, H.V. Volden, Vest. MGill, Set. Khim. 38 (1997) 297. [2] J.L. Alonso, R.M. Villamanan, J. Chem. Soc., Faraday Trans. II. 85 (1989) 137. [3] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AI-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres,

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