The molecular structure of benzene derivatives, part 2: 4-chloro-benzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 444 (1998) 47-56

The molecular structure of benzene derivatives, part 2: 4-chloro-benzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations Harald M011endal a, Snefrid Gundersen a, Maxim A. Tafipolsky b'*, Hans Vidar Volden a ~Departrnent of Chemistry, University of Oslo, Box 1033, Blindern, N-0315 Oslo, Norway hDepartment of Chemistry, Moscow State University, Moscow 119899, Russian Federation

Received 24 April 1997; revised 23 July 1997; accepted 4 August 1997

Abstract The molecular structure of gaseous 4-chlorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF/6-311G** level of theory. The planar C~ symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large amplitude motion. The most important structure parameters (rg) from the joint analysis with estimated total errors (in parentheses) are: (C-C)mean= 1.398(1) A, C-C1 = 1.734(3) A, C-C( = O) = 1.482(10) A, C = O = 1.216(5) A, (CCcIC = 121.0(5)°, and (CCcHoC = 120.2(8)°. A scaled molecular force field has been determined. The ground state rotational constants have been determined from microwave data. © 1998 Elsevier Science B.V. Keywords: Benzene derivatives; Electron diffraction; Microwave spectroscopy

1. Introduction The molecular structures of 2-chlorobenzaldehyde [1] and 3-chlorobenzaldehyde [2] have been determined by gas electron diffraction. In this work we are going to fulfil these series of chlorobenzaldehydes by presenting our results for 4-chlorobenzaldehyde. The interaction between the substituents through the benzene ring is of particular interest in such systems. Recently we have published the molecular structure of gaseous 4-fluorobenzaldehyde [3]. The shortening

* Correspondingauthor.

of the C - F bond distance was found there in comparison with that obtained previously for fluorobenzene in gas phase. The present work, as our previous one, is devoted to the study of the substitution effects in benzaldehyde derivatives using the Molecular Orbital Constrained method [2].

2. Ab initio calculations The numbering of atoms is shown in Fig. 1. Ab initio calculations were carried out with the Gaussian 94 program package [4] using a standard 6-311G** basis set at the Hartree-Fock level. A full geometry optimization constrained to C~ symmetry has been

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S0022-2860(97)00310-4

48

H. M¢llendal et al./Journal (~f Molecular Structure 444 (1998) 47-56

HI~ Hll

HI0

Ol3

cl

Table 1 Computed geometry at the HF/6-311G** level (distances in A., angles in degrees) of 4-chlorobenzaldehyde

H7

~

H8

Cl9 Fig. 1. Numbering of atoms for 4-chlorobenzaldehyde.

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 - 9 0 ° for the ((C2C jC 120 ~3 dihedral angle. The geometry has been optimized for each step assuming planar phenyl and formyl groups a n d C19 and C J2 atoms being in the phenyl plane. The planar equilibrium structure with Cs symmetry was found to be the only stable conformation (see Fig. 1).

Distance

Value

Angle

Value

C rC2 C2-C3 C3-C~ C4-C5 C5-C6 C6-CI C2-H7 C3-H8 Cs-Hi0 C6-HII C4-C19 Cb-Ci2 CI2-OI3 C 12-H 14

1.3907 1.3784 1.3864 1.3802 1.3844 1.3852 1.0738 1.0730 1.0729 1.0762 1.7407 1.4849 1.1836 1.0963

(C6-C ~-C2 (CI-C2-C3 (C2-C3-C4 (C3-C4-C5
119.68 120.31 119.06 121.58 118.74 120.62 119.15 119.26 120.31 120.02 124.38 114.89 120.73

The barrier to the perpendicular orientation of the formyl group ((C2CJC12013 = 90 °) was found to be 37 kJ/mol. The computed bond distances and valence angles of the equilibrium structure are listed in Table 1. The molecular force field of 4-chlorobenzaldehyde was calculated and used in a normal coordinate analysis. The force field calculation confirms that equilibrium structure has Cs symmetry.

8M(

' ' ' ;

....

1'0 . . . .

1'5 . . . .

210 . . . .

2'5 . . . .

3'0

s (A -i) Fig. 2. Experimental (dots), calculated (solid) and difference molecular intensity curves for 4-chlorobenzaldehyde.

49

11. M ¢ l l e n d a l et a l . / J o u r n a l o f M o l e c u l a r Structure 4 4 4 (1998) 4 7 - 5 6

3. Normal coordinate analysis The definition of the internal coordinates for 4-chlorobenzaldehyde is the same as shown in Fig. 2 in Ref. [3], and the symmetry coordinates are given in Table 2. Since the 4-chloro-benzaldehyde molecule has Cs symmetry, its 36 fundamentals are classified into the symmetry types as follows: I" = 25A' + 11A".

The ab initio force constants were scaled as suggested by Pulay et al. [5] in order to reproduce the experimental set of frequencies. The scale factors were optimized by minimizing the weighted mean square deviation between the observed frequencies (solution state) of 4-chlorobenzaldehyde [6] and the HF/6311G** calculated harmonic frequencies of the molecule with the ASYM40 program [7]. The scale factors

Table 2 Symmetry coordinates and the scaling factors for 4-chlorobenzaldehyde Description

Symmetry coordinate a

Scale factor b

S ~= R i $2 = R2 $3 = R3 S4 = R 4 $5 = R5 $6 = R6 $7 = R7 $8 = r8 $9 = r9 S~0 = r~0 S~l = r~] S~2 = rl2 Sl.~ = rl3 814 = r~4 S 15 = 0/i - a2 5 1 6 = 0 / 3 - 0/4 S]7 = 0/5 - 0/6 Sis = 0/7 - ~8 S~9 = 0/9 - ~]0 $2o = 0/ii - 0/12 $2~ = ~ 13 - 0/]4 $22 = ~ ~ 823 = fll -/32 + f13 - 134+/35 -f16 524 = 2ill - / 3 2 - / 3 3 + 2/34 -f15 --•6 $25 = ~ 2 -- ~ 3 + ~ 5 -- ~6

0.90 [ 1] 0.81 [21 0.81 [2] 0.81 [2] 0.81 [2] 0.8112] 0.8112] 0.83 [3] 0.83 [3] 0.92 [4] 0.83 [3] 0.83 [3] 0.72 [5] 0.83 [3] 1.06 [6] 0.82 [7] 0.82 [7] 0.83 [8] 0.82 [7] 0.82 [7] 0.83 [9] 0.83 [9] 0.84 [10] 0.84 [10]

$26 = at

0.77 0.71 0.76 0.76 0.71 0.76 0.76 1.30 0.80 0.80 0.80

As

C - C ( = O ) stretch C - C stretch in the ring C - C stretch in the ring C - C stretch in the ring C - C stretch in the ring C - C stretch in the ring C - C stretch in the ring C - H stretch in the ring C - H stretch in the ring C - C I stretch C - H stretch in the ring C - H stretch in the ring C - O stretch ( O = ) C - H stretch C - C ( = O ) asymmetric bend C - H asymmetric bend C - H asymmetric bend C - C I asymmetric bend C - H asymmetric bend C - H asymmetric bend H - C - O asymmetric bend H - C - O symmetric bend C - C - C in plane bend C - C - C in plane bend C - C - C in plane bend

0.84 [10]

A n

H - C - O out-of-plane bend C - C ( = O ) out-of-plane bend C - H out-of-plane bend C - H out-of-plane bend C - C I out-of-plane bend C - H out-of-plane bend C - H out-of-plane bend H - C = O torsion Ring torsion Ring torsion Ring torsion

$27 = 02 $28 = a3 Sz9 = a4

$30 = S~1 = S~2 = $3~ = $34 = S.~5 = $36 =

a5 06 ~7 7"7 r~ - r2 + 7-3 - r4+7-5-7-6

7"r - 7"3 + 7"4 - 7-6 - r l + 2r2 - r3 - r 4 + 2 r 5 - r 6

"Not normalized. ~"I'he group number in the refinement is in brackets (see text).

[1 I] [12] [13] [13] [12] [13] [13] [14] [15] [15] [15]

H. M¢llendal et a l./Journal of Molecular Structure 444 (1998) 4 7-56

50

Table Scaled A' 1

3 force constants

of 4-chlorobenzaldehyde 1

2

in symmetry 3

coordinates 4

a 5

6

7

8

4.836769

2

0.315239

6.071650

3

- 0.063609

0.767950

4

0.001254

- 0.608543

0.818849

6.134719

5

- 0.049107

0.478216

- 0.591275

0.814508

6

- 0.028884

- 0.559108

0.528241

- 0.599591

0.812187

6.236220

7

0.247772

0.743311

- 0.553176

0.486447

- 0.610017

0.775473

6.241218

8

- 0.041512

0.068291

0.053448

- 0.002258

- 0.011033

- 0.010381

- 0.005307

9

- 0.001011

- 0.002041

0.065758

0.043911

- 0.009102

- 0.012022

- 0.008370

0.008099

10

- 0.044254

- 0.064429

- 0.012612

0.376468

0.382297

- 0.009754

- 0.068999

0.006720

6.418223

6.329672

5.145455

11

- 0.001404

- 0.007244

- 0.012784

- 0.009578

0.044441

0.063361

- 0.002180

0.000599

12

- 0.015598

- 0.000352

- 0.013795

- 0.012224

- 0.004597

0.069114

0.077141

0.000920

13

0,794300

- 0.057045

0.038444

- 0.027963

- 0.031916

0.048752

- 0.055201

0.019725

14

0,080396

0.007425

0.003419

0.000966

- 0.008229

0.009933

0.018618

0.001042

15

0,058917

0.199316

0.042918

- 0.041926

0.040118

- 0.045014

- 0.192935

- 0.044216

16

0,010827

- 0.125396

0.135654

0.009615

- 0.021279

0.020731

- 0.012349

- 0.020044

17

- 0,004712

- 0.013838

- 0.137530

0.167741

0.012270

- 0.022103

0.018375

0.006104

18

- 0,000316

- 0.020413

0.014787

- 0.206247

0.207122

- 0.015093

0.018936

- 0.005460

19

0,005598

- 0.018108

0.021221

- 0.012173

- 0.166268

0.138586

0.013194

0.000329

20

- 0.008682

0.013737

- 0.020327

0.019954

- 0.013113

- 0.138588

0.126079

0.006076 - 0.026022

21

0.111179

- 0.044133

- 0.001184

0.007493

- 0.036371

0.021841

0.056530

22

- 0.325680

- 0.022604

- 0.002363

- 0.000990

0.031749

- 0.018693

- 0.030480

0.016488

23

- 0.176822

- 0.006416

- 0.003437

0.026051

0.045414

- 0.009809

- 0.011758

0.089613

24

- 0.169522

0.096888

- 0.224022

0.089210

0.105386

- 0.236243

0.102634

0.051976

25

- 0.017353

0.200508

- 0.006691

- 0.158914

0.164947

0.009951

- 0.208733

- 0.064352

9

10

11

12

13

14

15

16

9

5.142012

10

0.005371

3.930655

11

0.002376

0.004893

5.145859

12

0.000758

0.009933

0.008902

13

0.005831

0.045823

0.006813

0.005292

14

0.001835

0.007505

0.001264

0.014138

0.475336

4.441782

15

0.006679

- 0.000090

- 0.007570

0.028483

- 0.048176

0.047430

1.000558

16

- 0.005788

0.015004

0.000149

- 0.007975

- 0.019771

0.003953

0.009841

17

0.007307

- 0.011873

0.005240

- 0.000240

- 0.004345

- 0.002916

- 0.011650

0.008935

18

0.027850

0.003839

- 0.027710

0.007166

0.003413

- 0.001070

- 0.006609

- 0.010433

5.041541 11.505957

0.487817

19

- 0.005075

0.011664

- 0.007337

- 0.006059

0.006845

0.001176

- 0.013788

- 0.000443

20

- 0.000103

- 0.015449

0.005499

0.001685

- 0.012180

0.008929

- 0.002524

- 0.010952

21

- 0.001948

0.015976

0.003611

0.002814

0,300141

- 0.120044

- 0.099528

0.001466

22

- 0.001675

- 0.012262

- 0.002895

- 0.011945

0.218910

0.051710

- 0.003860

- 0.000756

23

- 0.090398

0.286902

- 0.090540

0.096265

- 0.035492

- 0.015749

0.000518

- 0.006324

24

0.048485

- 0.372439

0.049465

0.052233

- 0.058403

- 0.015781

0.000428

- 0.072766

25

0.065046

- 0.001499

- 0.064747

0.073152

- 0.006373

0.002470

0.075921

- 0.036440

17 17

18

19

20

21

22

23

24

0.495591

18

- 0.000516

0.854988

19

- 0.010980

- 0.000816

20

- 0.000503

- 0.010949

0.008867

0.502811

21

0.005244

0.000410

0.005092

0.000264

0.815427

22

0.001247

0.000823

- 0.003993

- 0.007865

- 0.214844

1.153154

23

- 0.001031

- 0.000354

0.000141

- 0.000362

- 0.004353

0.035036

1.229572

24

0.065151

- 0.000170

- 0.065498

0.065425

- 0.025063

0.048125

0.011842

1.300271

25

- 0.037087

0.064396

- 0.036698

- 0.039310

- 0.051081

0.021473

0.003557

0.004888

25

1.199830

25

0.494614

H. MOllendal et al./Journal of Molecular Structure 444 (1998) 47-56

51

Table 3 (continued) A" 26 27 28 29 30 31 32 33 34 35 36 34 35 36

-

-

26 0.375831 0.040140 0.001225 0.001210 0.001703 0.001654 0.008035 0.006234 0.018890 0.018420 0.004200 34 0.354725 0.012476 0.000065

27

28

0.400887 - 0.076010 - 0.002605 - 0.018496 0.000087 - 0.065664 - 0.014345 0.134851 0.139028 0.000609 35

0.462544 - 0.073211 - 0.007163 - 0.010239 0.002241 - 0.006161 - 0.147763 - 0.079810 0.138885 36

0.299835 - 0.001248

0.325191

29

30

0.400516 - 0.063808 0.001942 - 0.010602 0.005019 0.136030 - 0.063798 - 0.128497

0.506600 - 0.069788 - 0.004734 0.003533 - 0.147156 0.150000 - 0.004992

31

0,399349 - 0.067281 - 0.003954 0.136483 - 0.068196 0.125088

32

0.442823 0.008247 - 0.142434 - 0.078429 - 0.130933

33

0.075713 - 0.005898 - 0.005364 - 0.019241

aSee Table 2 for definition of the symmetry coordinates. Stretch: mdyn ~. i, bend, out-of-plane bend, torsion: mdyn ,&/rad2, stretch/bend: mdyn/rad. are g i v e n in T a b l e 2, a n d the s c a l e d f o r c e field c o r r e s p o n d i n g to the set o f the s y m m e t r y c o o r d i n a t e s is listed in T a b l e 3. T h e c o m p a r i s o n o f o b s e r v e d a n d c a l c u l a t e d f r e q u e n c i e s is g i v e n in T a b l e 4, T h e root m e a n s q u a r e s a m p l i t u d e s o f v i b r a t i o n (u), the p e r p e n d i c u l a r a m p l i t u d e c o r r e c t i o n c o e f f i c i e n t s (K), a n d the set o f h a r m o n i c c o r r e c t i o n s to the g r o u n d state rotational c o n s t a n t s (~iBvib) w e r e c a l c u l a t e d f r o m the s c a l e d f o r c e field at e x p e r i m e n t a l t e m p e r a t u r e (64°C). F o r e a c h t o r s i o n a l d e p e n d e n t d i s t a n c e w e u s e d i n t e r p o l a t e d v a l u e s o f the v i b r a t i o n a l a m p l i t u d e s , u(~b), w h i c h w e r e c a l c u l a t e d for ~b = 0, 30, 6 0 a n d 9 0 ° u s i n g the A S Y M 4 0 p r o g r a m . In this p r o c e s s w e u s e d n o s y m m e t r y b l o c k i n g for ~b = 0 ° in o r d e r to u s e the s a m e f o r c e field at the o t h e r t o r s i o n a l angles. T h e c o n t r i b u t i o n f r o m the f o r m y l g r o u p t o r s i o n m o d e was e x c l u d e d d u r i n g c a l c u l a t i o n s o f v i b r a t i o n a l a m p l i t u d e s ( ' 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 m o d e l .

f r e q u e n c i e s are b e l i e v e d to b e a c c u r a t e to w i t h i n 0.1 M H z . T h e r e s u l t s o f the a b initio c a l c u l a t i o n s indicate that 4 - c h l o r o b e n z a l d e h y d e is an a s y m m e t r i c r o t o r w i t h a s y m m e t r y p a r a m e t e r ~e ~ - 0.96, a n d w i t h d i p o l e m o m e n t c o m p o n e n t s #~ = - 0 . 6 2 8 D a n d #b = -- 0.533 D. T h e c o m p o n e n t a l o n g the c-axis is z e r o s i n c e the m o l e c u l e p o s s e s s e s a s y m m e t r y p l a n e a n d the c - a x i s is p e r p e n d i c u l a r to this plane. It is also p r e d i c t e d (see T a b l e 4) that t h e r e are s e v e r a l h e a v y a t o m low f r e q u e n c y v i b r a t i o n s (inp l a n e a n d o u t - o f - p l a n e n o r m a l m o d e s ) , w h i c h are the c a u s e o f rich m i c r o w a v e s p e c t r u m . T h e a - t y p e R - b r a n c h a n d b - t y p e Q - b r a n c h s p e c t r a o f the g r o u n d v i b r a t i o n a l state w e r e a s s i g n e d . T h e g r o u n d state r o t a t i o n a l c o n s t a n t s w i t h t h e i r u n c e r t a i n t e s (in p a r e n t h e s e s ) are g i v e n in the fifth c o l u m n in T a b l e 7. T h e i n e r t i a d e f e c t w a s o b t a i n e d to b e - 0 . 2 1 7 u~, 2.

5. Gas electron diffraction 4. Microwave spectrum and assignment T h e m i c r o w a v e s p e c t r u m w a s r e c o r d e d at r o o m t e m p e r a t u r e a n d at p r e s s u r e o f a f e w p a s c a l in the 2 6 . 5 - 3 4 G H z r e g i o n u s i n g the s p e c t r o m e t e r d e s c r i b e d in [8]. T w o S t a r k v o l t a g e s ( 5 0 V a n d 5 0 0 V ) w e r e a p p l i e d to m o d u l a t e the lines. T h e m e a s u r e d t r a n s i t i o n

G a s e l e c t r o n d i f f r a c t i o n data w e r e r e c o r d e d w i t h a B a l z e r s E l d i g r a p h K D G - 2 unit [9] w i t h a c o n v e n t i o n a l m e t a l inlet s y s t e m w i t h the n o z z l e t e m p e r a t u r e o f a b o u t 64°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 - p h o t o g r a p h i c p l a t e d i s t a n c e s o f 498.81 m m (five plates) a n d 248.81 m m (five plates), r e s p e c t i v e l y . T h e e l e c t r o n w a v e l e n g t h w a s 0 . 0 5 8 6 2 5 A. All plates w e r e s c a n n e d

52

H. MOllendal et al./Journal of Molecular Structure 444 (1998) 47-56

Table 4 Comparison of observed and calculated fundamental frequencies (cm L) of 4-chlorobenzaldehyde No.

Observed [6] ~ IR freq.

Calculated (HF/6-31 IG**) Raman freq.

Freq. b

Freq. ~

IR intensity (km mol-%

Raman activity amu-i)

3036 3032 3023 2995 2803 1789 1608 1581 1487 1404 1370 1287 1182 1156 1145 1072 1057 994 806 683 619 524 345 300 176

3071 3067 3057 3031 2836 1709 1613 1583 1495 1415 1385 1296 1210 1160 1152 1091 1058 1004 821 702 630 545 360 307 190

1.47 3.05 0.15 8.84 109.41 373.98 105.28 18.10 27.77 0.69 38.65 11.17 49.71 10.00 18.26 77.38 13.12 16.97 84.09 4.84 1.22 22.21 8.86 0.88 8.28

12&43 75.26 41,20 69.92 130.00 99.17 133.97 5.40 0.98 1.24 3.04 1.42 25.01 2.37 19.62 21.09 3.20 1.33 17.20 8.41 6.67 0.17 4.03 2.16 1.09

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

A' 3085 w 3068 w 3048 w 3032 w 2829 s 1710 vs 1595 s 1576 s 1486m 1415 w 1383m 1296m 1284 w 1209s 1162 s 1093 s 1082m 1014 s 834 s 699 w 626 w 542m 358m 307m 194 s

26 27 28

A" 1006 w 967 w 945 vw

992 959 926

1006 965 933

0.00 0.04 0.00

6.93 0.32 0.12

29 30 31

816 s 787w 699 w

816 797 704

820 802 699

7.26 54.54 0.05

0.21 0.03 0.02

32

481 m

477

477

26.22

0.20

33

(405)

420

421

0.24

0.04

34 35 36

243 w 180 s 104m

289 169 84

296 182 86

3.14 9.62 5.01

0.98 0.62 0.18

1697 1586 1478 1406

1200 1161 1088

832 695 622 538 358 307 193

P.E.D. d

(/~4

4958,4859 94511 5059,5058 96512 101Si4 85513 2353,2256,13524,1057 2152,2057,1955,1554 20517, 17519, 1252o, 1154 45522,1352t 40522,1453, llSi9 29517,225~9,195~6,1452o 3851,13523,1352o, 1156 2452o, 23519,2254,1652,1653,1555 32516,2557, 2355, 18517 3551o,1754,145~,12523 4456,3353,1254 55523 2852,1551,13524,1352t 32524,2051o, 1852t 7552~ 2351o, 12521,1251 33524,1451o, 145m13518 51518,10524 505t5,235~8,18521

84526 63528, 37529, 15532, - 18($36, $28)e 56532, 4653b 24534, 135:8, - 26($34, $32), - 24($34, $30 59529, 33528, 1553t, 1 IS32 5553t, 40532, 19529, 13535, 11528 195534, 2853o, 20527, - 5 1 ( 5 3 4 , 53o), --45(534, 527), -- 21(534, 528), -- 21($34, Ss2), - 20($34, 529), 18($34, $31) 1065~5, 6153o, 45527, - 62($35, $3o), - 55(S~5, $27) 147536, - 23($36, $29), - 23($36, S]t), 22($36, $32), - 21($36, $28) 37527, 21530, 17533, 10536 59533, 16535, 1453o 33535, 25527, 18533, 23($35, $27) -

243 180

~Solution values. bScaled by factor 0.81. CScaled by a number of factors listed in Table 2. OContribution larger than 10% is given.

-

H. Mollendal et al./Journal of Molecular Structure 444 (1998) 47-56

on an Agfa Arcus II scanner and the data processed with a program system written by T.G. Strand [10]. Atomic scattering factors were taken from Ref. [11 ]. Backgrounds were drawn using a nineth (498.81 mm) and tenth (248.81 mm) degree polynomials. The resulting modified molecular intensity curves are shown in Fig. 2.

6. Structure refinement Structure refinements were carried out with the program KCED26 [12], based on a molecular model of Cs symmetry for ~b -- 0 ° (see Fig. 1). Due to very large correlations between some of the geometrical parameters we had to make constraints from ab initio HF/6-311G**, even if the rotational constants were incorporated into the analysis. The following constraints and assumptions are made: 1. planar phenyl and formyl groups for all values of 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 C - H bond distances are all different, and their differences are fixed to the ab initio calculated values. Thus, only one C - H distance was refined; 4. all the CCH valence angles were fixed to their values calculated by ab initio; and 5. the C-C1 bond distance is directed along the bisector of the (C3C4C 5 angle. A joint structural analysis based upon the electron diffraction data and the rotational constants for 4-chlorobenzaldehyde 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 ° were calculated using the formula: 0

2

r~ - r a = u v / r -

3 / 2 a 3 ( u 2 - u 2) - Ko

where u0 is the root mean squares amplitude of vibration, and K0 is the perpendicular amplitude correction at T = 0 K, respectively, and a3 is the cubic anharmonicity constant which is estimated to be 2.0 ,~-i for C - C , C - C I , and C - O bond distances, and 2.6 ,~-~ for

53

C - H bond distances. The asymmetry parameters, r, for bonded atomic pairs were estimated from the diatomic approximation by r 1/6a3 u4. The r values for all the non-bonded distances were ignored. Correction for shrinkage was incorporated in the analysis by refining a geometrically consistent r~ structure. The uncertainties in the Bz rotational constants have been estimated to be 10% of the vibrational correction tSBvib = B z - Bo. Centrifugal distortions and corrections arising from electronic contributions were neglected. 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 (~b) ranging from 0 to 90 ° in steps of 6 °. The mole fraction of each pseudo-conformer was assumed proportional to the Boltzmann factor exp( - V ( ~ ) / R T ] . The potential energy V(~b) was assumed to have the form: =

V(~b) = 0.5 V + 2(1 - cos2~b) where V2 is the barrier height of the internal rotational in the molecule.

7. Results and discussion Least squares refinements of 11 geometrical parameters: C ~ - C : , CI-Cj2, CI2-O13, C - C I , and C - H bond distances, valence angles (CtC2C3, (C2C3C4, (C3C4C5, (C2CIC12, (CIC12OI3 and (C tC 12H t4 with the constraints mentioned above converged to the values listed in Table 5. The vibrational amplitudes were fixed to their calculated values except those given in Table 6. The refinements were carried out using a diagonal weight matrix. The estimated standard deviations have been doubled to reflect uncertainties due to data correlation, and further expanded to include an estimated scale uncertainty of 0.1%. Two correlation coefficients C IC2C3/ C2C3C 4 = - 0.95 and r(C I - C z ) / r ( C j - C 1 2 ) = - 0.76 exceed the value of 0.7. The difficulties in determination of the internal rotational barrier in benzaldehyde and in its derivatives have been discussed in the literature [13]. Our ab initio computations (HF/6311G**) on 4-chlorobenzaldehyde predict the barrier height to 37 kJ/mol. Refinement of the barrier height gives the value 29 kJ/mol with standard deviation of 87 kJ/mol. A number of refinements assuming the barrier heights to be in the range 2 0 - 4 0 kJ/mol were

54

H. M¢llendal et al./Journal of Molecular Structure 444 (1998) 47-56

Table 5 Results of electron diffraction least squares refinement (rg) of 4chlorobenzaldehyde (distances in ,~,,angles in degrees) Parameter C 1-C2 AC2C3 AC3C4 AC4C5 ACsCe ACsC1 C-CI C1-C12 C12-O13 (C-H)mea n LCI-C2-C 3 LC2-C3-C4 LC3.C4-Cs ZC2-C1.C12 LC1-C12-O13 LC1.C12-H14

C2-C3 C3-C4 C4-C 5 C5-C6 C6-C 1 CI2-H14 LC6-C 1-C 2 LC;4-Cs-C6 /C5-C6-C 1 LC5-C4-Ct 9 R-factors, % 50 cm 25 cm

GED GED + MW Independent parameters 1.404(1)a 1.404(1) [-0.0123]b [-0.0043] [-0.0105] [-0.0063] [-0.0055] 1.733(2) 1.734(3) 1.480(9) 1.482(10) 1.214(4) 1.216(5) 1.092(6) 1.094(7) 121.6(9) 121.8(12) 118.0(8) 117.6(9) 120.8(4) 121.0(5) 120,0(6) 119.7(8) 126.0(10) 125.5(12) 118(4) 116(6) Dependent parameters 1.392 1 1.392 1 1.400 [ 1.400 (1) 1.394 j~ (1) 1.394 1.398 / 1.398 1.398 J 1.399 1.114(6) 1.117(7) 119.9(7) 120.2(8) 121.0(8) 121.2(9) 118.6(8) 118.2(9) 119.6(4) 119.5(5) 3.2 4.8

3.3 4.9

aEstimatedtotal errors in parentheses: a : [2(aus)2 + (0.001r)2]I/2 (see text). bValues in brackets were unrefined and they correspond to differencesfrom ab initio HF/6-31 IG** results. tested. The values of the barrier in this range fitted the experimental data equally well and did not influence the values of other parameters significantly. This

shows that there is not enough information in the electron diffraction data to determine the barrier height for 4-chlorobenzaldehyde, and the barrier height was therefore fixed to the value calculated by ab initio. Experimental and calculated intensity curves are compared in Fig. 2. Experimental and calculated radial distribution curves are compared in Fig. 3. The rotational constants are listed in Table 7. Comparing the third and fifth columns in Table 7 we can see the fairly good agreement between microwave and electron diffraction data. W e have to note (see Table 6) that the calculated vibrational amplitudes (u) are significantly smaller than the experimental ones. Table 8 shows some structure parameters for chlorine substituted benzene derivatives. The C-C1 bond distance found in 4-chlorobenzaldehyde appears to be the same, within experimental error, as in chlorobenzene and significantly shorter than that obtained in 4-chloronitrobenzene (see Table 8). This is probably due to the larger electronegativity of the nitro group in comparison with that of formyl group. 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 C1 atom) is larger than 120 ° (in benzene itself). It is worthwhile to note that the C = O and C - C ( = O) bond distances and the CC(CHO)C valence angle found in 4-chloro-benzaldehyde appear to be the same, within experimental error, as in benzaldehyde itself (see Table 8),

Acknowledgements W e are grateful to the Norwegian Academy of Science and Letters, to the Russian Foundation for Basic Researches (Project No. 96-03-32660a) for financial support and to the Norwegian Research Council (Programme for Supercomputing) for a generous grant of computing time. The authors wish to thank Professors S. Samdal and L.V. Vilkov for fruitful discussions of this work.

H. MOllendal et al./Journal of Molecular Structure 444 (1998) 47-56

55

Table 6 Some vibrational amplitudes of 4-chlorobenzaldehyde from the joint analysis of electron diffraction and microwave data (in ,%)" Type

r9

U,x p

Ucalc b

Type

r9

u,x p

ucal¢ b

1.094 1.117 1.734

1 0.088~.(10) 0.091) 0.052 (4)

0.077 0.080 0.048

Independent parameters

(C-C)me=n

1.398 1.482 1.216

0.052[ 0.053 f (3) 0.043

CI...C 3 C2...C4

2.44 2.39

0.074 0.074

C3...C5 C4...C6

2.43 2.43

0.074 0.074

CI...C 5 C2...C6 C2...C12 C6...C12 C1...O13 C 1,..C4 C2...C5 C3...C6 C3...CI9 C5...CI9

2.40 2.43 2.50 2.50 2.40 2.78 2.76 2.84 2.71 2.71

0.074 0.074 0.083 0.084 0.076 0.077 0.078 0.078 0.080 0.080

C1"C12 C=O

0.047 0.049 0.038

(C-H)rr~n C12"H14 C-CI

Dependent parameters 0.056 C2...CIQ 0.056 C6...CI9

(3)

(4)

3.99 4.02

0.085 ~ 0.085 J, (6)

0.064 0.084 0.065 0.085

0.056 0.056

C3...C12 C5...C12

3.78 3.76

0.083 ~ 0.083 J (14)

0,055

C 4.,.C12

0.056 0.065 0.085 0.058 0.062 0.063 0.083 0.005 0.085

CI...CI 9 C12,..Cl9 02..,013 C3...013 04...O13 C5...O13 C6...O13 CI9..,013 CI...H 7

4.26 4.51 5.99 2.89 4.28 5.05 4.79 3.63 6.76 2.16

[0.088]" 0.082 (11) 0.064 0.110 (18) 0.070 [0.099] [0.100] [0.086] [0.087] [0.063] 0.136 (20) 0.091 0,131 (10) 0.100

~Values in brackets were unrefined.bCalculated from the scaled force field (see text).

:(r)

=,? ~ ~,

~

~- ~- o o u~ u d uu

6

6

A 0

2

4 r

8

8

(•)

Fig. 3. Experimental (dots), calculated (solid) and difference radial distribution curves for 4-chlorobenzaldehyde. An artificialdamping constant of ~ = 0.0025 ,~2 was used.

56

H. Mollendal et al./Journal of Molecular Structure 444 (1998) 47-56

Table 7 Rotational constants of 4-chlorobenzaldehyde (MHz) Type

GED"

GED + MW

MW ~

MW d

MW e

A B C

5092(25) 690(2) 608( 1)

5062.2(2) ° 691.99(2) 608.77( ! )

5058.25 691.9960 608.8328

5056.99(2) 691.976(6) 608.846(6)

5062.1 (5) 691.94(5) 608.84(5)

~Calculated from the geometry above (see column 2 in Table 5). 820" in parentheses. ~Taken from [14] (CI35C6H4COH). dThis work. eThe same as in column 5 but with harmonic corrections: 6B~h = 0 . 5 ~ i - - B : - B 0 (see text).

Table 8 Comparison of some structure parameters (rg) in benzene derivatives ~ Parameter (C-C) ..... C-CI C-O C-C(=O) (CCc~C (CC cHoC

C6H5C1 1.400(1) 1.737(5)

1.395(4) 1.739(2)

C6HsCHO

p - C1C6H4CHO

p - CIC6H4NO2

1.397(3)

1.388(4) 1.710(7)

I 19.9(7)

1.398(1) 1.734(3) 1.216(5) 1.482(10) 121.0(5) 120.2(8)

[ 17]

This work

1.212(3) 1.479(4) 121.7(6)

121.6(2)

(CC NO2C

Ref.

[ 15]

[ 16] h

120.4(1.0) 123.2(1,6) [ 18]

"Distances in ~,, angles in degrees. br°u structure.

References [1] L. Sch~ifer, S. Samdal, K. Hedberg, J. Mol. Struct. 31 (1976) 29. [2] N.S. Chiu, J.D. Ewbank, M. Askari, L. Scb~ifer, J. Mol. Struct. 54 (1979) 185. [3] S. Samdal, T.G. Strand, M.A. Tafipolsky, L.V. Vilkov, M.V. Popik, H.V. Volden, J. Mol. Struct., in press. [4] 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, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94 Program, Revision B.3, Gaussian, Inc., Pittsburgh, 1995. [5] G. Fogarasi, P. Pulay, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 14, Elsevier, Amsterdam, 1985, pp. 125219. [6] J.H.S. Green, D.J. Harrison, Spectrochim. Acta, Part A 32 (1976) 1265.

[71 L. Hedberg, I.M. Mills, J. Mol. Spectrosc. 160 (1993) 117. [81 G.A. Guirgis, K.-M. Marstokk, H. M¢llendal, Acta Chem. Scand. 45 (1991) 482. [9] W.Zeil,J. Haase, L. Wegmann, Z. Instrumentenkd. 74 (1966) 84. [10] S. Gundersen, T.G. Strand, J. Appl. Cryst. 29 (1996) 638. [ I 1] A.W. Ross, M. Fink, R. Hildebrandt, International Tables for X-ray Crystallography, Vol. C, Kluwer, Dordrecht, 1992. [12] B. Andersen, G. Gundersen, S. Samdal, H.M. Seip, T. Strand, Program description Department of Chemistry, University of Oslo, Oslo, 1980. [13] T. Schaefer, R. Sebastian, F.E. Hruska, J. Mol. Struct. (Theochem) 281 (1993) 269. [14] R.K. Kakar, E.A. Rinehart, Symp. Mol. Struct. and Spectr., Ohio, 1970. [15] N.P. Penionzhkevich, N.I. Sadova, L.V. Vilkov, J. Struct. Chem. (Russia) 20 (1979) 446. [16] S. Cradock, J.M. Muir, D.W.H. Rankin, J. Mol. Struct. 220 (1990) 205. [17] K.B. Borisenko, C.W. Bock, I. Hargittai, J. Phys. Chem. 100 (1996) 7426. [18] N.I. Sadova, N.P. Penionzhkevich, L.V. Vilkov, J. Struct. Chem. (Russia) 17 (1976) 652.