Conformational stability of CH3CH2P(Z)F2(Z=O,S) from temperature dependent FT-IR spectra of rare gas solutions and ro structural parameters

Journal of Molecular Structure 516 (2000) 131–152 www.elsevier.nl/locate/molstruc

Conformational stability of CH3CH2P(Z)F2 (Z ˆ O,S) from temperature dependent FT-IR spectra of rare gas solutions and ro structural parameters J.R. Durig*, J.B. Robb II, J. Xiao 1, T.K. Gounev Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA Received 4 March 1999; accepted 6 April 1999

Abstract Variable temperature (from 255 to 21508C) studies of the infrared spectra (3500–400 cm 21) of ethylphosphonic difluoride, CH3CH2P(O)F2 and ethylphosphonothioic difluoride, CH3CH2P(S)F2 dissolved in liquid xenon or krypton have been recorded. From these data, the enthalpy differences have been determined to be 76 ^ 9 cm 21 (0.91 ^ 0.11 kJ/mol), for CH3CH2P(O)F2 with the trans conformer the more stable rotamer and 53 ^ 7 cm 21 (0.63 ^ 0.08 kJ/mol) for CH3CH2P(S)F2 but with the gauche conformer the more stable form. Complete vibrational assignments are presented for both molecules, which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G(d) calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/ 6-31G(d) and/or MP2/6-31G(d) ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with some corresponding results for some similar molecules. The ro adjusted structural parameters have been obtained for both molecules from a combination of the microwave rotational constants and ab initio predicted parameters. The corresponding ro structural parameters have been obtained for some similar molecules for comparison. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Conformational stability; FT-IR spectra; Rare gas solutions; Ethylphosphonic difluoride; Ethylphosphonothioic difluoride

1. Introduction We [1] recently carried out a temperature dependent infrared study of ethyldifluorophosphine dissolved in liquid krypton to determine the conformational stability. From this investigation, it was clear * Corresponding author. Tel.: 1 1-8816-235-1136; fax: 1 1816-235-5191. E-mail address: [email protected] (J.R. Durig). 1 Taken in-part from the dissertation of Jinpeng Xiao, which will be submitted to the Department of Chemistry of the University of Missouri-Kansas City, Kansas City, MO in partial fulfillment of the PhD degree.

that the trans conformer (methyl group trans to the nonbonded electron pair on the phosphorous atom) is more stable than the gauche form by 80 ^ 7 cm 21 (0.96 ^ 0.24 kJ/mol). The value agrees with the earlier reported [2] value of 56 ^ 22 cm 21 (0.67 ^ 0.26 kJ/mol) with the trans conformer more stable from a Raman and infrared investigation of the gas. This result indicates that the conclusion from the microwave study [3] that the gauche form is more stable in the vapor is in error. As a continuation of these studies, we were interested in what effect the substitution for the non-bonded lone pair by an oxygen or sulfur atom would have on the

0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00194-5

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J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

values when appropriate. The calculations have been carried out with several basis sets at both the restricted Hartree–Fock (RHF) and with full electron correlation by the perturbation method (Moller–Plesset) to second order [7]. Additionally, the ro structural parameters have been determined for both molecules from the combined microwave data and ab initio calculations. The results of this experimental and theoretical study are reported herein.

2. Experimental

Fig. 1. Comparison of experimental and calculated infrared spectra of ethylphosphonic difluoride: (a) infrared spectrum of the sample dissolved in liquid krypton; (b) mixture of the calculated trans and gauche conformers with the experimentally determined DH of 76 cm 21; (c) pure gauche; and (d) pure trans. The bands marked with an asterisk are due to an impurity.

conformational stability. Therefore, we have carried out variable temperature studies of the infrared spectra of ethylphosphonic difluoride, CH3CH2P(O)F2, and ethylphosphonothioic difluoride, CH3CH2P(S)F2 dissolved in liquid xenon or krypton to determine the conformational stabilities and the enthalpy differences between the conformers. Although, the conformational stabilities of these molecules [4–6] have been confidently determined for the liquids and solids, there are still questions on their stabilities in the vapor state. To support the spectroscopic studies, we have carried out ab initio calculations to predict the conformational stabilities and structural parameters. Additionally, we have calculated the vibrational frequencies and the infrared and Raman intensities along with the Raman depolarization values. These quantities have been compared to the experimental

The samples of ethylphosphonic difluoride, CH3CH2P(O)F2, and ethylphosphonothioic difluoride, CH3CH2P(S)F2 were prepared by the fluorination of ethylphosphonic dichloride (Aldrich Chemical Co, Milwaukee, WI) and ethylphosphonothioic dichloride (Strem Chemicals Inc., Newburyport, MA), respectively, with freshly sublimed antimony trifluoride (Aldrich), dispersed in “glass wool”. The products were separated by trap-to-trap distillation and purified on a low-temperature, low-pressure fractionation column. The purified samples were contained in sample tubes equipped with greaseless stopcocks and stored under vacuum in a refrigerator at 258C. The mid-infrared spectra of the samples dissolved in liquified xenon or krypton as a function of temperature (Figs. 1(a) and 2(a)) were recorded on a Bruker IFS-66 Fourier transform interferometer equipped with a globar source, a Ge/KBr beamsplitter, and a DTGS detector. The temperature studies ranged from 255 to 21008C or 2105 to 21508C and were performed in a specially designed cryostat cell consisting of a 4 cm pathlength copper cell with wedged silicon windows sealed to the cell with indium gaskets. The complete system is attached to a pressure manifold to allow for the filling and evacuation of the cell. The cell is cooled by boiling liquid nitrogen and the temperature is monitored by two Pt thermoresistors. Once the cell is cooled to the desired temperature, a small amount of sample is condensed into the cell. The system is then pressurized with xenon or krypton gas, which immediately starts condensing in the cell, allowing the compound to dissolve. For each temperature investigated, 100 interferograms were recorded at 1.0 cm 21 resolution, averaged, and transformed with a boxcar truncation

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Fig. 2. Comparison of experimental and calculated infrared spectra of ethylphosphonothioic difluoride: (a) infrared spectrum of the sample dissolved in liquid xenon; (b) mixture of the calculated trans and gauche conformers with the experimentally determined DH of 53 cm 21; (c) pure trans; and (d) pure gauche.

function. The observed fundamental bands for the trans and gauche conformers are listed in Tables 1 and 2 for ethylphosphonic difluoride and ethylphosphonothioic difluoride, respectively.

3. Ab initio calculations The LCAO-MO-SCF RHF calculations were performed with the Gaussian-94 program [8] using Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were obtained by

133

the simultaneous relaxation of all of the geometric parameters using the gradient method of Pulay [9]. The structural optimizations for both the trans and gauche conformers were carried out with initial parameters taken from those previously reported from the microwave studies [5,6] of CH3CH2P(O)F2 and CH3CH2P(S)F2. The 6-31G(d) and 6-311 1 G(d,p) or 6-31111G(d,p) basis sets were employed at the level of RHF and/or Moller–Plesset (MP2) to second order [7]. The predicted structural parameters for CH3CH2P(O)F2, and CH3CH2P(S)F2, are listed in Tables 3 and 4, respectively. In order to obtain a more complete description of the molecular motions involved in the normal modes of these molecules we have carried out a normal coordinate analysis. The force fields in Cartesian coordinates were calculated by the Gaussian-94 program [8] with the MP2/6-31G(d) basis set. Internal coordinates (Fig. 3) were used to calculate the G and B matrices using the structural parameters given in Tables 3 and 4. Using the B matrix, the force field in Cartesian coordinates was then converted to a force field in internal coordinates, and the pure ab initio vibrational frequencies were reproduced. The force constants for the trans and gauche conformers can be obtained from the authors. Subsequently, scaling factors of 0.9 for stretchings and bendings and 1.0 for the torsional coordinates, and the geometric average of scaling factors for interaction force constants were used to obtain the fixed scaled force field and resultant wavenumbers. A set of symmetry coordinates was used (Table 5) to determine the corresponding potential energy distributions (PEDs). A comparison between the observed and calculated frequencies for the two molecules along with the calculated infrared intensities, Raman activities, depolarization ratios and PEDs are given in Tables 1 and 2. Theoretical infrared (Figs. 1 and 2) and Raman (Figs. 4 and 5) spectra were predicted using fixed scaled wavenumbers and infrared intensities determined from the MP2/6-31G(d) calculations and Raman scattering activities and depolarization values determined from the RHF/6-31G(d) basis set. Infrared intensities were calculated based on the dipole moment derivatives with respect to the Cartesian coordinates. The derivatives were taken from the ab initio calculations and transformed to normal

134

Table 1 Observed and calculated frequencies (cm 21) and PED for ethylphosphonic difluoride Species

n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 n 16 n 17 n 18 n 19 n 20 n 21 n 22 n 23 n 24 n 25 n 26 n 27

A 00

a

Fundamental

CH3 antisymmetric stretch CH2 symmetric stretch CH3 symmetric stretch CH3 antisymmetric deformation CH2 deformation CH3 symmetric deformation PyO stretch CH2 wag CH3 rock CC stretch PF2 symmetric stretch CP stretch PF2 wag CPO bond PF2 deformation CCP bend CH3 antisymmetric stretch CH2 antisymmetric stretch CH3 antisymmetric deformation CH2 twist CH3 rock PF2 antisymmetric stretch CH2 rock PF2 rock PF2 twist CH3 torsion Ethyl torsion

Trans

Gauche

Ab initio

Fixed scaled b

IR int.

Raman act. c

Calc. dep. c

Obs. gas d

Xenon soln.

PED

Ab initio

Fixed scaled b

IR int.

Raman act. c

Calc. dep. c

3220 3131 3129 1572 1520 1480 1403 1347 1108 1034 908 729 491 414 329 187 3228 3186 1566 1315 1087 932 777 405 286 218 79

3055 2971 2968 1491 1442 1404 1331 1278 1051 981 861 692 466 393 312 177 3062 3022 1486 1248 1032 884 738 386 279 211 79

9.4 10.5 5.5 10.0 11.6 1.0 123.9 74.8 11.8 5.9 165.9 16.7 47.4 28.0 5.8 1.0 7.9 0.0 8.9 1.8 21.2 165.4 1.6 19.0 1.3 0.2 1.0

76.53 112.93 78.18 12.19 13.58 1.79 3.33 3.08 3.85 7.95 0.78 9.86 1.31 3.42 1.56 0.12 21.42 98.30 19.0 6.47 0.92 1.46 0.20 1.33 0.18 0.09 0.00

0.70 0.10 0.02 0.74 0.75 0.74 0.32 0.21 0.45 0.45 0.30 0.04 0.74 0.42 0.48 0.64 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

2965 2941 2906 1470 1415 1395 (1342) 1281 1049 983 888 713 496 426 333 (185) (2992) 2956 (1460) 1242 1031 896 752 407 (292) (201) (76)

2971 2928 2893 1461 1408 1389 1355 1274 1045 983 882 712 490 425 – – 2990 2951 1461 1237 1024 – 749 405 – – –

100S1 75S2,24S3 76S3,24S2 92S4 99S5 98S6 39S7,42S8 48S8,35S7 45S9,43S10 41S10,23S9,22S7 68S11,11S12 57S12,23S11 74S13,20S16 40S14,38S15,11S12 53S15,23S16,12S14 42S16,36S14,13S13 96S17 96S18 92S19 61S20,29S21 36S21,35S20,20S23 87S22 62S23,21S22 67S24,22S25 46S25,48S26 39S26,30S25,19S24,12S23 97S27

3219 3133 3126 1571 1522 1479 1382 1370 1103 1030 906 740 486 398 298 173 3225 3191 1565 1310 1082 938 792 421 322 214 73

3053 2972 2965 1490 1444 1403 1312 1300 1047 978 859 702 461 379 288 165 3059 3027 1484 1243 1027 890 753 400 306 209 73

10.0 5.3 9.1 10.7 8.8 2.5 124.8 55.0 15.3 4.0 188.4 18.5 45.2 11.7 0.7 1.6 7.0 0.4 8.9 6.0 8.5 171.4 10.2 30.3 8.6 0.4 0.5

58.56 99.32 92.60 11.37 14.12 1.73 3.24 1.24 3.90 7.28 0.83 9.76 1.90 0.55 0.26 0.08 29.92 107.56 18.48 7.01 2.59 2.06 0.80 2.58 1.44 0.17 0.07

0.68 0.13 0.02 0.74 0.75 0.75 0.23 0.27 0.29 0.51 0.04 0.04 0.75 0.67 0.75 1.66 0.73 0.73 0.75 0.73 0.62 0.68 0.36 0.50 0.43 0.75 0.75

For Convenience, the vibrations are ordered according to the trans conformer. Scaled ab initio calculations with factor 0.9 for all coordinates except the torsions at the MP2/631G(d) level. c Obtained from the RHF/6-31G(d) basis set. d Wavenumbers in parentheses taken from Ref. [4]. b

Obs. gas d 2963 – – – – – 1366 1287 1046 980 – 720 493 389 (298) (180) 2996 – – 1232 1024 897 760 426 324 (192) (68)

Xenon soln.

PED

– – – – – – 1349 1282 1038 – 876 719 490 – – – – – – 1015 904 754 425 – – –

689S1,29S17 95S2 95S3 91S4 98S5 98S6 64S8,16S7 58S7,20S8 46S9,40S10 46S10,22S9,21S7 72S11,12S12 56S12,18S11 27S14,23S13,18S16,18S24 21S13,38S15,12S24 33S15,27S26,16S14 45S16,22S15,18S24,12S14 65S17,32S1 93S18 90S19 54S20,29S21 34S21,35S20,19S23 90S22 59S23,24S21 69S24,35S13 50S25,15S15,23S16 50S26,21S14 92S27

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

A0

Vib. No a

Table 2 Observed and calculated frequencies (cm 21) and PED for ethylphosphonothioic difluoride Species

n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 n 16 n 17 n 18 n 19 n 20 n 21 n 22 n 23 n 24 n 25 n 26 n 27

A 00

Fundamental

CH3 antisymmetric stretch CH2 symmetric stretch CH3 symmetric stretch CH3 antisymmetric deformation CH2 deformation CH3 symmetric deformation CH2 wag CH2 rock C–C stretch PF2 stretch C–P stretch PyS stretch PF2 symmetric wag PF2 deformation CCP bend CPS bend CH3 antisymmetric stretch CH2 antisymmetric stretch CH3 antisymmetric deformation CH2 twist CH3 rock PF2 antisymmetric stretch CH2 rock PF2 rock PF2 twist CH3 torsion Ethyl torsion

Trans

Gauche

Ab initio

Fixed b scaled

IR int. c

Raman act. d

Calc. dep. c

Obs. gas d

Xenon soln.

PED

Ab initio

Fixed scaled b

IR int.

Raman act. c

Calc. dep. c

3217 3123 3127 1569 1517 1476 1364 1102 1033 927 835 640 376 286 429 165 3222 3186 1564 1313 1087 910 784 349 270 206 77

3052 2963 2966 1488 1440 1401 1294 1045 980 879 792 607 358 278 407 156 3057 3022 1484 1246 1032 863 745 332 257 200 77

9.4 5.5 8.3 9.0 7.1 4.4 7.3 13.0 5.8 270.8 135.8 3.5 24.1 2.3 9.2 0.7 7.0 0.4 10.8 4.5 53.4 134.6 2.5 14.7 2.4 0.1 0.2

71.97 76.06 128.70 10.32 14.75 1.84 0.47 3.17 6.42 2.05 1.75 15.99 2.17 2.69 2.77 1.47 23.24 109.91 18.61 6.10 1.74 1.30 1.30 3.72 2.02 0.77 0.34

0.67 0.04 0.10 0.74 0.75 0.72 0.49 0.33 0.54 0.47 0.54 0.09 0.60 0.74 0.74 0.66 0.67 0.74 0.74 0.74 0.75 0.25 0.28 0.48 0.70 0.73 0.75

2993 2902 2932 1466 1414 (1390) 1281 1044 984 (885) 807 621 376 (277) 429 (170) 2998 2963 2963 1241 1030 879 743 357 (262) (183) (71)

2988 2890 2922 1458 1406 1385 1274 104 982 893 804 619 – – 429 – 2988 2949 2949 1236 1019 873 741 – – – –

89S1,10S17 79S2,21S3 78S3,21S2 91S4 100S5 98S6 70S7,12S8 044S8,42S9 39S9,28S7,23S8 61S10,23S12 40S11,18S12,17S10 45S12,30S11,14S10 61S13,17S14 25S14,44S26,11S16 32S15,18S14,18S24,17S16 30S16,37S15,16S24,13S25 85S17,11S1 94S18 94S19 71S20,29S21 33S21,38S20 90S22 74S23,21S21 25S24,30S14,19S13 72S25,15S24 37S26,25S16 91S27

3217 3128 3126 1571 1517 1476 1371 1371 1032 434 820 662 434 371 295 165 3229 3183 1563 1314 1082 906 775 320 262 216 75

3052 2968 2966 1490 1439 1401 1300 1300 979 412 778 628 412 352 280 157 3063 3019 1483 1247 1027 859 736 309 251 210 75

8.3 18.1 0.4 8.8 10.9 2.2 7.7 7.7 4.7 24.6 133.3 8.3 24.6 14.9 11.7 0.2 7.1 0.0 9.5 0.5 7.7 131.8 0.7 1.9 1.6 0.2 0.4

90.94 91.84 99.45 12.79 13.18 2.12 0.09 0.09 7.93 5.31 1.75 12.49 5.31 4.59 4.45 1.59 24.49 85.64 20.88 6.04 0.79 1.47 0.15 2.98 1.73 0.19 0.14

0.70 0.13 0.03 0.74 0.71 0.75 0.16 0.16 0.44 0.33 0.29 0.08 0.33 0.44 0.54 0.72 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

Obs. gas d 2993

(1390) 1366 1287 1046 429 788 647 429 367 (288) (170) 2998 2998 – 1232 1022 879 760 (313) (262) (187) (73)

Xenon soln.

PED

2988 2890 2887 1458 1406 1385 1274 1274 982 429 785 646 429 – – – 2988 2988 1458 1236 1019 870 731 – – – –

99S1 70S2,30S3 70S3,30S2 92S4 100S5 98S6 72S7,11S8 72S7,11S8 37S9,28S7,25S8 57S13,21S15,16S12 43S11,30S12 28S12,31S11,22S10 57S13,21S15,16S12 76S14,15S16 36S15,25S13,17S16,12S14 58S16,27S15 97S17 97S17 92S19 73S20,28S21 38S21,41S20,11S23 90S22 81S23,22S21 62S24,33S26 74S25,19S26 41S26,24S24,23S25 98S27

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

A0

Vib. No a

a

For convenience, in comparison with the CH3CH2P(O)F2 molecule the vibrations are ordered according to the trans conformer. Scaled ab initio calculations with factor 0.9 for all coordinates except the torsions at the MP2/6-31G(d) level. c Obtained from the RHF/6-31G(d) basis set. d Wavenumber in parentheses taken from Ref. [6]. b

135

136 Table 3 ˚ , bond angles in degrees, rotational constnats in MHz, and dipole moments in Debye), rotational constants, dipole moments and energy for Structural parameters (bond distance in A CH3CH2P(O)F2 Parameter

a b

MP2/6-31G(d)

MP2/6-311 1 G(d,p)

Microwave a (ro) Trans

Trans

Gauche

Trans

Gauche

Trans

Gauche

1.792 1.533 1.083 1.083 1.083 1.085 1.085 1.438 1.550 1.550 115.0 109.6 111.3 111.3 108.1 108.1 108.3 111.1 111.1 106.4 106.3 106.3 118.1 104.1 104.1 98.60 114.8 114.8 180.0 180.0 128.6 2128.6 4.29 0.14 0.00 4.29 4329 2336 2277 0.217840 29

1.791 1.535 1.083 1.084 1.083 1.085 1.085 1.438 1.551 1.549 112.5 109.7 111.2 111.0 108.2 108.4 108.4 110.6 111.2 107.3 106.8 108.2 118.6 103.9 103.7 98.75 114.6 114.9 178.7 302.6 128.5 2128.7 2.95 2.90 0.33 4.15 4279 2276 2236 0.217970

1.789 1.528 1.092 1.091 1.091 1.094 1.094 1.471 1.583 1.583 114.2 109.9 111.0 111.0 108.2 108.2 108.3 111.3 111.3 106.6 106.4 106.4 118.7 103.2 103.2 98.42 115.2 115.2 180.0 180.0 128.9 2128.9 4.38 0.39 0.0 4.40 4329 2336 2277 1.146185 12

1.787 1.530 1.091 1.092 1.091 1.094 1.093 1.471 1.583 1.582 111.4 110.0 110.9 110.7 108.2 108.5 108.4 110.9 111.6 107.5 106.9 108.3 118.9 103.1 103.4 98.67 115.0 115.4 178.4 302.9 128.4 2129.2 3.04 2.86 0.78 4.25 4118 2269 2221 1.146238

1.786 1.531 1.092 1.091 1.091 1.094 1.094 1.462 1.582 1.582 114.7 109.6 111.1 111.1 108.2 108.2 108.4 111.5 111.5 106.8 106.0 106.0 119.2 103.4 103.4 98.6 114.8 114.8 180.0 180.0 128.8 2128.8 4.68 0.62 0.00 4.72 4215 2312 2266 1.588594 5

1.785 1.802 ^ 0.012 1.532 1.522 1.091 1.091 1.092 1.091 1.092 1.091 1.094 1.093 1.093 1.093 1.463 1.440 1.582 1.549 ^ 0.013 1.580 1.549 ^ 0.013 111.8 113.6 ^ 1.0 109.6 110.0 111.1 111.0 110.9 111.0 108.1 108.4 108.5 108.4 108.4 108.0 111.0 111.3 111.6 111.3 107.7 107.6 106.4 106.3 108.0 106.3 119.0 118.6 103.3 104.4 ^ 0.4 103.5 104.4 ^ 0.4 98.8 98.8 114.6 114.2 115.0 114.2 178.9 180.0 303.0 180.0 128.3 128.4 2129.0 2128.4 3.46 2.89 0.74 4.57 4142 4336 2262 2344 2220 2288 1.588616

This study b (roadj ) Gauche 1.795 ^ 0.013 1.522 1.091 1.091 1.091 1.093 1.093 1.440 1.546 ^ 0.008 1.546 ^ 0.008 111.8 ^ 1.2 110.0 111.0 111.0 108.4 108.4 108.0 111.3 111.3 107.6 107.3 107.3 118.6 103.6 ^ 0.8 103.6 ^ 0.8 99.3 114.7 114.7 178.4 302.0 128.4 2128.4 2.649(55) 2.476(61) 0.000 3.6262(32) 4278 2292 2252

Trans 1.777 1.531 p 1.092 p 1.091 p 1.091 p 1.094 p 1.094 p 1.452 1.549 1.549 114.7 p 109.6 p 111.1 p 111.1 p 108.2 p 108.2 p 108.4 p 111.5 p 111.5 p 106.8 p 106.0 p 106.0 p 118.5 104.4 104.4 98.3 114.6 114.6 180.0 p 180.0 p (128.4) (2128.4)

4336 2345 2289

From Ref. [5]. Parameters marked with asterisks ( p ) are kept fixed at the ab initio MP2/6-311 1 G(d,p) values. Values in parentheses are dependent parameters.

Gauche 1.776 1.532 1.091 1.092 1.092 1.094 1.093 1.452 1.549 1.548 111.8 p 109.6 p 111.1 p 110.9 p 108.1 p 108.5 p 108.4 p 111.0 p 111.6 p 107.7 p 106.4 p 108.0 p 118.0 104.5 104.2 98.3 114.6 114.7 178.9 p 303.0 p (128.7) (2128.6)

4277 2294 2249

p p p p p p

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

r(P–C1) r(C1– C2) r(C2– H1) r(C2– H2) r(C2– H3) r(C1– H4) r(C1– H5) r(PyO) r(P–F1) r(P–F2) \PC1C2 \H1C2C1 \H2C2C1 \H3C2C1 \H1C2H2 \H1C2H3 \H2C2H3 \H4C1C2 \H5C1C2 \H4C1H5 \PC1H4 \PC1H5 \OPC1 \F1PC1 \F2PC1 \F1PF2 \OPF1 \OPF2 t (H1C2C1P) t (OPC1C2) t (F1PC1O) t (F2PC1O) |ma| |mb| |mc| |mt| A B C 2 (E 1 693) DE (cm 21)

RHF/6-31G(d)

Table 4 ˚ , bond angles in degrees, rotational constnats in MHz, and dipole moments in Debye), dipole moments and total energies for ethylphoStructural parameters (Bond distance in A sphonothioic difluoride Parameter

a b

MP2/6-31G(d)

MP2/6-311 1 G(d,p)

Microwave a (ro)

This study b (roadj )

Trans

Gauche

Trans

Gauche

Trans

Gauche

Trans

Gauche

1.802 1.534 1.084 1.083 1.083 1.085 1.085 1.899 1.558 1.558 114.6 109.3 111.4 111.4 108.1 108.1 108.4 111.1 111.1 106.4 106.4 119.5 102.9 102.9 98.14 115.3 115.3 180.0 180.0 50.20 250.80 4.166 1.409 0.000 4.398 3860 1648 1549 0.851122 34

1.801 1.533 1.083 1.084 1.083 1.085 1.085 1.899 1.557 1.558 113.6 109.4 111.2 111.3 108.1 108.3 108.5 110.9 111.0 106.9 106.7 120.5 102.0 102.4 98.32 115.2 115.1 180.8 261.16 176.4 53.98 4.075 0.790 1.012 4.273 2747 2049 1600 0.851278

1.797 1.528 1.092 1.091 1.091 1.095 1.095 1.892 1.593 1.593 114.2 109.7 111.0 111.0 108.3 108.3 108.4 111.3 111.3 106.4 106.4 119.7 102.1 102.1 97.80 116.0 116.0 180.0 180.0 50.40 250.40 4.212 1.266 0.000 4.198 3778 1642 1551 1.726620 92

1.796 1.529 1.092 1.092 1.092 1.094 1.095 1.892 1.592 1.594 112.2 109.9 110.8 111.0 108.2 108.4 108.4 111.2 111.3 107.1 107.1 120.2 101.9 101.5 97.94 116.1 115.8 180.7 260.08 176.1 55.75 3.920 0.461 1.134 4.106 2672 2073 1604 1.727042

1.794 1.530 1.092 1.091 1.091 1.094 1.094 1.888 1.592 1.592 114.5 109.5 111.2 111.2 108.2 108.2 108.4 111.4 111.4 106.0 106.0 120.5 1020 102.0 97.67 115.6 115.6 180.0 180.0 50.4 250.4 3.568 2.080 0.000 4.130 3806 1639 1552 2.256117 91

1.793 1.531 1.092 1.092 1.092 1.094 1.094 1.889 1.591 1.592 112.5 109.6 111.1 111.0 108.2 108.4 108.4 111.1 111.4 106.5 107.0 120.7 101.9 101.7 98.01 115.6 115.4 180.2 59.8 170.4 69.4 3.873 0.056 1.232 4.064 2687 2064 1605 2.25630

1.814 ^ 0.011 1.532 ^ 0.005 1.093 1.093 1.093 1.091 1.091 1.861 ^ 0.007 1.563 ^ 0.008 1.563 ^ 0.008 114.9 ^ 0.2 110.50 110.50 110.50 108.42 108.42 108.42 110.48 110.48 105.78 105.78 119.0 ^ 0.7 101.1 ^ 0.5 101.1 ^ 0.5 97.40 117.31 117.31 180.0 180.0 49.96 249.96

1.800 1.532 1.093 1.093 1.093 1.095 1.095 1.880 1.555 1.555 112.6 110.0 110.0 110.0 108.99 108.99 108.99 110.5 110.5 106.84 106.84 119.4 102.0 102.0 98.43 115.94 115.94 180.0 56.94 173.78 72.34

3874 1664 1564

2712 2118 1625

^ 0.007 ^ 0.006 ^ 0.003 ^ 0.003 ^ 0.003 ^ 0.008 ^ 0.008 ^ 0.003 ^ 0.005 ^ 0.005 ^ 0.3 ^ 0.2 ^ 0.2 ^ 0.2

^ 0.7 ^ 0.7

^ 0.5 ^ 0.2 ^ 0.2

^ 0.20

Trans

Gauche

1.798 1.534 1.092 1.092 1.092 1.094 1.094 1.877 1.562 1.562 114.3 111.3 110.6 110.6 108.4 108.4 107.2 111.6 111.6 105.2 105.2 119.5 102.5 102.5 97.5 115.8 115.8 180.0 180.0 (50.4) (250.4)

1.797 1.534 1.091 1.093 1.093 1.094 1.094 1.878 1.561 1.562 111.9 109.0 110.6 110.5 108.1 108.3 108.2 110.3 110.6 106.4 106.9 119.9 102.3 102.2 97.8 115.8 115.6 181.6 257.0 (173.1) (72.2)

3874 1664 1564

2712 2118 1625

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

r(P–C1) r(C1– C2) r(C2– H1) r(C2– H2) r(C2– H3) r(C1– H1) r(C1– H2) r( PyS) r(P–F1) r(P–F2) \PC1C2 \C1C2H1 \C1C2H2 \C1C2H3 \H1C2H2 \H1C2H3 \H2C2H3 \C2C1H1 \C2C1H2 \H1C1H2 \PC1H2 \C1PS \C1PF1 \C1PF2 \F1PF2 \SPF1 \SPF2 t (PC1C2H1) t (SPC1C2) t (F1PC1C2) t (F2PC1C2) |ma| |mb| |mc| |mt| A B C 2 (E 1 1015) DE (cm 21)

RHF/6-31G(d)

From Ref. [6]. Values in parentheses are dependent parameters. 137

138

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152 Table 5 Symmetry coordinates for CH3CH2P(O)F2 and CH3CH2P(S)F2 Species Description a

Symmetry coordinate b

A0

S1 S2 S3 S4

Fig. 3. Internal coordinates of ethylposphonic difluoride and ethylphosphonothioic difluoride.

coordinates by: !   X 2m u 2mu ˆ Lij ; 2Qi 2Xj j where Qi is the ith normal coordinate, Xj the jth Cartesian displacement coordinate, and Lij the transformation matrix between the Cartesian displacement coordinates and normal coordinates. The infrared intensities were then calculated by: "      # 2my 2 Np 2m x 2 2mz 2 1 1 : Ii ˆ 2 2Qi 2Qi 2Qi 3c The predicted infrared spectra of the trans and gauche conformers of CH3CH2P(O)F2 and CH3CH2P(S)F2 are shown in Figs. 1(c) and (d) and 2(c) and (d), respectively, with the mixtures of the two conformers shown in Figs. 1(b) and 2(b). The calculated spectra are in reasonably good agreement with the mid-infrared spectra of the samples dissolved in liquified krypton (Fig. 1(a)) and xenon (Fig. 2(a)), which demonstrates the utility of the predicted spectra for identifying the fundamental modes for the two different conformers. The evaluation of the Raman activity by using the analytical gradient methods has been developed [10,11]. The activity Sj can be expressed as Sj ˆ gj …45a2j 1 7b2j †; where gj is the degeneracy of the vibrational mode j, a j the derivative of the isotropic polarizability and b j that of the anisotropic polarizability. The Raman

A 00

CH3 antisymmetric stretch CH3 symmetric stretch CH2 symmetric stretch CH3 antisymmetric deformation CH3 symmetric deformation CH2 deformation CH2 wag C–C stretch CH3 rock PF2 stretch CP stretch PyO (S) stretch PF2 wag PF2 deformation CCP bend CPO (S) bend CH3 antisymmetric stretch CH2 antisymmetric stretch CH3 antisymmetric deformation CH2 twist CH3 rock PF2 antisymmetric stretch CH2 rock PF2 twist PF2 rock CH3 torsion Asymmetric torsion

ˆ 2r12 r22r3 ˆ r1 1 r2 1 r3 ˆ r4 1 r5 ˆ 2a 12a 22a 3

S5 ˆ a 1 1 a 2 1 a 32b 12b 22b 3 S6 ˆ 4d 2p 12p 22e 12e 2 S7 ˆ p 1 1 p 22e 12e 2 S8 ˆ R S9 ˆ 2b 12b 22b 3 S10 ˆ x 1 1 x 2 S11 ˆ T S12 ˆ Q S13 ˆ f 1 1 f 22m12m2 S14 ˆ 4h 2f 12f 22m12m2 S15 ˆ u S16 ˆ g S17 ˆ r22r3 S18 ˆ r42r5 S19 ˆ a 22a 3 S20 S21 S22 S23 S24 S25 S26 S27

ˆ p 22p 12e 2 1 e 1 ˆ b 22b 3 ˆ x 12x 2 ˆ p 22p 12e 2 1 e 1 ˆ f 12f 22m1 1 m2 ˆ f 1 1 f 2 1 m12m2 ˆ t1 ˆ t2

a Symmetry coordinates are listed in order for the CH3CH2P(S)F2 molecule. b Not normalized.

scattering cross sections, dsj =sV; which are proportional to the Raman intensities, can be calculated from the scattering activities and the predicted wavenumbers for each normal mode using the relationship [12,13] 0 1 ! ! C 2s j … n0 2 nj † 4 24 p4 B h B C  C ˆ S; B 2hcnj A 8p2 cnj j sV 45 @ 12exp kT where n 0 is the exciting wavenumber, n j the vibrational wavenumber of the jth normal mode, and Sj the corresponding Raman scattering activity. To obtain the polarized Raman scattering cross sections,

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

139

Fig. 4. Raman spectra of ethylphosphonic difluoride: (a) experimental spectrum of the liquid; (b) calculated spectrum of the mixture of both conformers with a DH of 76 cm 21; (c) pure trans; and (d) pure gauche.

the polarizabilities are incorporated into Sj by Sj ‰…12rj †=…1 1 rj †Š where r j is the depolarization ratio of the jth normal mode. The Raman scattering cross sections and calculated wavenumbers from the scaled force constants obtained from the standard Gaussian program [8] were used together with a Lorentzian function to obtain the calculated spectra. The predicted Raman spectra of the pure trans and gauche conformers of CH3CH2P(O)F2 and CH3CH2P(S)F2 are shown in Figs. 4(c) and (d) and 5(c) and (d), respectively. In Figs. 4(b) and 5(b) the mixtures of the two conformers are shown with the experimentally determined DH values of 76 cm 21 with the trans conformer the more stable rotamer for CH3CH2P(O)F2, and 53 cm 21 with the gauche conformer the more stable rotamer for CH3CH2P(S)F2. The experimental Raman spectra of the liquids are shown in Figs. 4(a) and 5(a) for

Fig. 5. Raman spectra of ethylphosphonothioic difluoride: (a) experimental spectrum of the liquid; (b) calculated spectrum of the mixture of both conformers with a DH of 53 cm 21; (c) pure trans; and (d) pure gauche.

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J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

comparative purposes and the agreement is quite good.

4. Conformational stability There are several fundamentals for both of these molecules which show conformer doublets in the infrared and Raman spectra of the fluid phases. For ethylphosphonic difluoride, the P–C stretches are observed at 713 (trans) and 719 cm 21 (gauche) and are sufficiently separated to obtain the enthalpy difference between the conformers. Fundamentals are predicted for the trans conformer at 491, 414 and 405 cm 21 with similar modes for the gauche conformer predicted at 486, 421 and 398 cm 21, respectively. However, as only three bands are observed at 490, 425 and 405 cm 21 in the infrared spectra of the xenon and krypton solutions, it is concluded that the individual bands are not resolved for these conformer doublets. The CH2 rocks are also expected to produce a doublet with the gauche conformer having the higher frequency. The fundamental at 749 cm 21 has a high frequency shoulder and the wavenumber for this mode for the trans conformer in the infrared spectrum of the solid is 750 cm 21. Therefore, we assign the band to the trans conformer and the shoulder to the gauche rotamer. The only other region where doublets are observed is the PyO stretching and CH2 wag, but these modes have a greater chance of interference from combination bands. For ethylphosphonothioic difluoride, the ab initio calculations predict the separation between the conformational doublets of the P–C stretch and PyS stretch to be 15 and 22 cm 21, respectively. The P–C stretch is observed at 785 (trans) and 804 cm 21 (gauche) whereas the PyS stretch is observed at 646 (trans) and 619 cm 21 (gauche), giving separations of 19 and 27 cm 21, respectively. Other fundamentals are predicted for the trans conformer at 736 and 434 cm 21 with corresponding wavenumbers of 745 and 429 cm 21 for the gauche modes. However, the observed bands at 731 and 741 cm 21 are weak and unresolved whereas only one band is observed at 429 cm 21. The remaining fundamentals of the two conformers are near coincident and, thus, cannot be used for the conformational analysis.

In order to gain information about the enthalpy difference between the two conformers for both of these molecules, variable temperature studies in liquified xenon and krypton were carried out. The samples were dissolved in liquified xenon or krypton and the spectra were recorded at different temperatures varying from 255 to 21508C. Only small interactions are expected to occur between the dissolved sample and the surrounding xenon or krypton atoms and, consequently, only small frequency shifts are anticipated when passing from the gas phase to the liquified noble gas solutions [14–18]. Thus, the resulting spectra (Figs. 1(a) and 2(a)) should be representative of the infrared spectra of the vapor phases. A significant advantage of this type of temperature study is that the conformer bands are better resolved in comparison with those in the infrared spectrum of the gas. This is particularly important as most of the conformer bands for these molecules are predicted to be observed within a few wavenumbers of each other. The best separated bands in the infrared spectra of the xenon and krypton solutions for both molecules are those assigned to the P–C stretch (Figs. 6 and 7). The higher frequency band in each doublet for each of these molecules has been confidently assigned to the gauche rotamer. Ten sets of spectral data were obtained for the 720/712 cm 21 conformer pair of the CH3CH2P(O)F2 molecule as well as for the CH3 rock at 1038/1045 cm 21. The intensities of the infrared bands were measured as a function of temperature and their ratios were determined. By applying the van’t Hoff equation 2ln K ˆ (DH/RT)2 (DS/R), where DS is the entropy change, we have determined DH from a plot of 2ln K versus 1/T, where DH/R is the slope of the line and K is the appropriate intensity ratio. It is assumed that DH is not a function of temperature. From the plot of the natural logarithm of the ratio of intensities as a function of the reciprocal of the absolute temperature (Table 6), an average DH of 76 ^ 9 cm 21 (913 ^ 109 J/mol) with the trans conformer the more stable form was determined for ethylphosphonic difluoride. Similarly, the pair of conformational bands at 785 (trans) and 804 (gauche) cm 21 belonging to the P–C stretch of ethylphosphonothioic difluoride (Table 7) gives a DH value of 51 ^ 8 cm 21 (610 ^ 96 J/mol), but the gauche conformer is the more stable form of this molecule. An additional

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Fig. 6. Temperature dependent infrared spectra of ethylphosphonic difluoride in the PC stretching region.

Fig. 7. Temperature dependent infrared spectra of ethylphosphonothioic difluoride in the PC stretching region.

141

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J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Table 6 Temperature and intensity ratios for the conformational study of ethylphosphonic difluoride, CH3CH2P(O)F2 in liquid xenon T (8C)

T 1000/T I719/I712 ln k (K) (K)

255 260 265 270 275 280 285 290 295 2100 DH a (cm 21)

218 213 208 203 198 193 188 183 178 173

4.587 4.695 4.808 4.926 5.051 5.181 5.319 5.464 5.618 5.780

1.592 1.606 1.600 1.570 1.553 1.537 1.521 1.486 1.442 1.398

0.465 0.474 0.470 0.451 0.440 0.430 0.420 0.396 0.366 0.335 78 ^ 7

I1039/I1045 ln k

1.287 1.367 1.289 1.286 1.252 1.225

0.252 0.310 0.254 0.251 0.224 0.203 70 ^ 30

Table 7 Temperature and intensity ratios for the conformational study of ethylphosphonothioic difluoride, CH3CH2P(S)F2 in liquid xenon T (8C)

I619/I646 T 1000/T I785/I804 2ln I785/I804 (K) (K)

255 260 265 270 275 280 285 290 295 2100 DH a (cm 21)

218 213 208 203 198 193 188 183 178 173

4.587 4.695 4.808 4.926 5.051 5.181 5.319 5.464 5.618 5.780

0.395 0.393 0.386 0.388 0.388 0.381 0.383 0.376 0.372 0.354

0.928 0.933 0.952 0.946 0.946 0.965 0.959 0.978 0.990 1.038 51 ^ 8

2ln I619/I646

1.661 1.611

20.508 20.477

1.592

20.465

1.582

20.459

1.496

20.403 55 ^ 13

Average value is 76 ^ 9 cm 21 (913 ^ 109 J/mol) with the trans conformer the more stable species.

Average value is 53 ^ 7 cm 21 (634 ^ 84 J/mol) with the gauche conformer the more stable species.

pair of bands (619/646 cm 21) which had a much poorer signal-to-noise ratio was used in the enthalpy determination of CH3CH2P(S)F2 which gave a value of 55 ^ 13 cm 21 (658 ^ 156 J/mol) with the gauche conformer more stable. The average DH value for these two determinations is 53 ^ 7 cm 21 (151 ^ 19 cal/mol) with the gauche conformer the more stable rotamer. We also attempted to conduct temperature studies in liquified krypton, but the sample began to crystallize at temperatures below 21158C. Also, there were problems with the decomposition of the ethylphosphonic difluoride sample in the liquid xenon and krypton solutions, which lead to interference with some of the conformer doublets. Therefore, these conformer pairs such as the ones at 1282/1274 cm 21 were omitted.

temperatures and sharpness of the lines attained in the spectra of xenon and krypton solutions make it easier to observe the splitting of fundamentals between the two conformers. Therefore, we will only discuss the changes in the assignments that result from the observation of additional fundamentals in the noble gas solutions and any reassignments of the fundamentals due to our normal coordinate calculations, which utilize ab initio force constants from a higher level of calculation. Complete assignments of the observed vibrational fundamentals of CH3CH2P(O)F2 and CH3CH2P(S)F2 are presented in Tables 1 and 2, respectively, along with the PEDs for the trans and gauche conformers. The initial investigation of ethylphosphonic difluoride utilized RHF ab initio calculations using the 3-21G(d) and 6-31G(d) basis sets with scaling factors of 0.9 for the stretching, 0.8 for the bending, and 1.0 for the torsional modes [4]. The current study employed the 6-31G(d) basis set with full electron correlation at the MP2 level [7] and scaling factors of 0.9 for the stretching and bending coordinates and 1.0 for the torsional modes. In the investigations of CH3CH2P(O)F2, significant mixing was found for the normal modes of both the trans and gauche conformers, especially for the low frequency vibrations. As these modes are strongly mixed, the assignment order of several fundamentals depends upon the PEDs resulting from the different levels of the ab initio

a

5. Vibrational assignment Complete vibrational assignments were provided in earlier studies of ethylphosphonic difluoride [4,5] and ethylphosphonothioic difluoride [6]. However, the normal coordinate calculations of CH3CH2P(S)F2 were based upon the transfer of force constants from the methylphosphonothioic difluoride and ethyldifluorophosphine molecules, whereas those for CH3CH2P(O)F2 utilized ab initio (RHF/3-21G(d)) calculated force constants. Additionally, the reduced

a

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

calculation. Thus, PED obtained from MP2/6-31G(d) calculations indicate a reversal in the assignment of three pairs of fundamentals compared to the previously reported ones [4]; the PyO stretch with the CH3 symmetric deformation, the CC stretch with the CH3 rock, and the PF2 deformation with that of the CPO bend. In addition, four bands at 1349, 1038, 972, and 904 cm 21 belonging to the PyO stretch, CH3 rock, CC stretch, and PF2 antisymmetric stretch of the gauche conformer, and one band at 1274 cm 21 belonging to the CH2 wag of the trans rotamer were assigned as a result of the improved resolution of the bands from the infrared spectrum of the sample dissolved in the liquified noble gas solutions. For ethylphosphonothioic difluoride, three pairs of fundamentals were reassigned according to the PED obtained from MP2/6-31G(d) ab initio calculations. A reversal in the assignments from that previously reported [6] is suggested for the CH3 rock with the CC stretch, the CCP bend with the CPS bend, and the PF2 rock with the PF2 twist. As a result of the reassignments and calculated vibrational frequencies, the band at 1022 cm 21 previously assigned [6] to the CC stretch of the gauche conformer is now assigned to the CH3 rock (n 22) of this conformer. The PF2 antisymmetric stretch previously assigned to a single band at 879 cm 21 is now assigned in the noble gas solution to the bands at 873 and 870 cm 21 for the trans and gauche conformers, respectively. Additionally, the higher resolution of the infrared spectrum of the gas phase allowed the CH3 rock (n 21) of the trans conformer to be observed as a Q-branch at 1030 cm 21. The remaining vibrational assignments are consistent with those previously reported for CH3CH2P(O)F2 [4,5] and CH3CH2P(S)F2 [6].

6. Discussion The potential surfaces governing internal rotation about the P–C bonds of ethylphosphonic difluoride and ethylphosphonothioic difluoride were obtained by calculating the molecular energies for a number of values of the OPC1C2 and SPC1C2 dihedral angles while keeping all other structural parameters fixed at the optimized values for the trans conformers of each molecule, respectively. At the maxima of the potential surfaces, corresponding to the structures in which the

143

C1 –C2 bonds are in the synperiplanar and anticlinal positions with respect to the PyO and PyS bonds, the energies were maximized with respect to the OPC1C2 and SPC1C2 dihedral angels and minimized with respect to all other structural parameters. In this way, the gauche to gauche and gauche to trans potential barriers of 661 cm 21 (7.86 kJ/mol) and 863 cm 21 (10.27 kJ/mol), respectively, were obtained for the CH3CH2P(O)F2 molecule with the MP2/631111G(d,p) basis set (Table 8). Similar barriers of 631 cm 21 (7.51 kJ/mol) and 1068 cm 21 (12.71 kJ/ mol) were obtained with the MP2/6-31G(d) basis for the gauche to gauche and gauche to trans potential barriers of the CH3CH2P(S)F2 molecule (Table 8). The potential surfaces obtained with both the MP2/631G(d) and MP2/6-31111G(d,p) basis sets are shown in Figs. 8 and 9 for ethylphosphonic difluoride and ethylphosphonothioic difluoride, respectively. In order to determine the potential parameters for the function governing the internal rotation around the C–P bond experimentally, we fitted the far infrared transitions of the P(O)F2 and P(S)F2 torsions to a potential function of the form: V…f† ˆ

n X

…Vi =2†…12cosif†:

iˆ1

The internal rotation constant, F(f ), was also represented as a Fourier series in terms of the torsional dihedral angle, f : F…f† ˆ Fo 1

n X

Fi cosif;

iˆ1

where the coefficients, Fi, were determined using the optimized structural parameters for the trans and gauche conformers using the ab initio MP2/6311G(d) level of calculation for each molecule. The relaxation of the structural parameters B(a ) during the internal rotation is incorporated into the above equation by assuming them to be small periodic functions of the torsional angle of the general type: B…a† ˆ a 1 b cos a 1 c sin a: Values are calculated at 158 intervals and the computer program employed, was developed in our laboratory. Four torsional transitions of the trans conformers and three torsional transitions of the gauche conformers

144

Potential constants

V1 V2 V3 V4 V6 DH (cm 21) c Trans/gauche barrier Gauche/ gauche barrier Gauche/trans barrier Dihedral angle a

Ethylphosphonothioic difluoride

Ethylphosphonic difluoride

Ref. [6]

MP2/631G(d)

Fit a

MP2/631111G(d,p)

Fit b

Ref. [4]

MP2/631G(d)

Fit a

MP2/631111G(d,p)

Fit b

253 ^ 6 138 ^ 5 744 ^ 26

257 ^ 2 221 ^ 3 858 ^ 3 27 ^ 1 216 ^ 1 253 ^ 8 833

2357 99 886 121 2297 2103 965

231 ^ 1 254 ^ 2 860 ^ 2 15 216 ^ 1 253 ^ 7 823

235 ^ 6 203 ^ 4 661 ^ 1

296 93 797 27

274 ^ 3 164 ^ 4 683 ^ 1 16 ^ 1

2127 122 796 20

270 ^ 3 158 ^ 5 679 ^ 1 17 ^ 1

63 ^ 37 819

268 249 903 26 2165 293 845

120 ^ 11 808

212 837

72 ^ 8 800

9 872

72 ^ 9 793

612

927

854

631

882

506

712

533

661

533

756

938

886

1068

876

688

849

728

863

721

56.94

60.08

60.10

60.96

61.00

57.35

57.10

57.09

57.25

57.23

Calculated using the assignments in Table 9 and Fourier coefficients (Fi) calculated from the MP2/6-31G(d) geometry. Calculated using the assignments in Table 9 and Fourier coefficients (Fi) calculated from the MP2/6-311 1 G(d,p) geometry. c A negative value indicates that the gauche form was calculated to be more stable. b

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Table 8 Potential function coefficient v for the asymmetric torsion of ethylphosphonic difluoride and ethylphosphonothioic difluoride

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

145

Fig. 8. Potential function of the asymmetric torsion of ethylphosphonic difluoride as determined by the MP2/6-31G(d) basis set (given by the dashed line) and the MP2/6-31111G(d,p) basis set (given by the solid line).

were used in the least squares fit (Table 9) for each of the molecules studied to obtain the first four Vi coefficients for ethylphosphonic difluoride and the first four Vi coefficients and the sixth coefficient of ethylphosphonothioic difluoride. Additionally, the enthalpy differences between the trans and gauche conformers obtained from the infrared spectra of the samples dissolved in liquified xenon and the torsional dihedral angles of the gauche conformers obtained from the ab initio calculations were also included in the fit. The

results obtained for the potential coefficients and barriers to internal rotation are summarized in Table 8 and are compared to those obtained from the ab initio calculations. Comparisons between the potential functions of the asymmetric torsions in this study and those previously reported [4,6] are also of some interest as seen in Table 8, the calculated potential barriers of CH3CH2P(O)F2 are very similar to those obtained previously [4], but the V1 and V2 potential constants

Fig. 9. Potential function of the asymmetric torsion of ethylphosphonothioic difluoride as determined by the MP2/6-31G(d) basis set (given by the dashed line) and the MP2/6-31111G(d,p) basis set (given by the solid line).

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Table 9 Observed (far infrared data from Ref. [4] for CH3CH2P(O)F2 and Ref. [6] for CH3CH2P(S)F2) and calculated transitions and splittings for the asymmetric internal rotation in ethylphosphonic difluoride and ethylphosphonothioic difluoride Conformer

Trans

Gauche

a b

Ethylphosphonic difluoride

Ethylphosphonothioic difluoride

Transition

Observed

Obs.2 Calc.

1←0 2←1 3←2 4←3 1←0 2←1 3←2

75.72 73.62 71.64 69.56 67.48 65.18 63.20

20.07 20.13 0.03 0.18 0.02 20.15 0.12

a

Obs.2 Calc 20.11 20.14 0.04 0.22 0.01 20.15 0.13

b

Transition

Observed

Obs.2 Calc. a

Obs.2 Calc b

1←0 2←1 3←2 4←3 1←0 2←1 3←2

73.25 71.72 69.93 68.15 73.62 72.24 70.82

0.05 0.08 20.06 20.08 20.05 20.05 0.07

0.00 0.06 20.04 20.03 20.02 20.02 0.04

Calculated using the fitted potential constants listed in Table 8 for the MP2/6-31G(d) geometry. Calculated using the fitted potential constants listed in Table 8 for the MP2/6-31111G(d,p) geometry.

are quite different. This is because the ab initio calculated dihedral angle and the experimental DH values were used in the earlier fit and they are highly correlated. The enthalpy difference determined from the noble gas infrared study is considerably lower than that determined previously [4], and the use of the calculated dihedral angle essentially fixes the V2 term. From the potential coefficients of CH3CH2P(S)F2 given in Table 8, it is evident that the V2 and V3 terms from our calculations are at variance with those previously reported [6]. One notable difference is that the dihedral angles calculated at the MP2/631G(d) and MP2/6-31111G(d,p) levels are 3–48 larger and this difference affects the V2 term. In the previous investigation of the potential function the authors [6] calculated a large positive V2 term, whereas in our calculations, a negative V2 term is obtained. Such a large difference affects the value of the V3 term and results in the gauche to gauche and gauche to trans potential barriers being larger than the previously reported ones [6], and also larger than the respective barriers determined for the CH3CH2P(O)F2 molecule. Although the electronegativity of the oxygen atom is higher than that of sulfur, oxygen also has a smaller van der Waal’s radius. On the basis of steric interactions, there is an increase in the steric hindrance as the methyl group eclipses the sulfur atom when going from one gauche conformer to the other as opposed to that encountered from the oxygen atom. The calculated PED of ethylphosphonic difluoride indicates all of the vibrational modes of the trans and

gauche conformers above 1400 cm 21 to be relatively pure modes. However, as predicted from the MP2/631G(d) calculations, many of the vibrational modes below 1400 cm 21 have significant mixing. With the exception of A 00 CH3 rocking motion, each fundamental of the trans conformer is calculated to have a contribution of approximately 40% or more from its descriptive assignment. Many of the fundamental modes of the gauche conformer, however, are calculated to consist of complex mixtures with contributions from several other low wavenumber modes. The calculated PED of ethylphosphonothioic difluoride is similar to CH3CH2P(O)F2 in that the fundamentals above 1200 cm 21 are relatively pure modes, but the mixing of the modes below 1200 cm 21 for the trans conformer consist of complex mixtures like those of the gauche conformer. It is interesting to note that the PF2 rock and PF2 deformation of the gauche conformer have a greater percentage of contribution from other vibrations than of their own descriptive assignment whereas these modes for the trans conformer are in different species and are relatively pure. A similar phenomenon occurs in the gauche conformer of ethylphosphonic difluoride, but the mixing consists of approximately equal contributions between the PF2 rocking and PF2 wagging modes. The predicted Raman and infrared wavenumbers, intensities and activities of ethylphosphonic difluoride are in good agreement with a few exceptions. The calculated Raman intensities of the CP stretching and CH2 waging vibrations are lower in intensity

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Fig. 10. Comparison of the effect of the DH value of ethylphosphonic difluoride on the calculation of the theoretical spectra in the region of the PC Stretch: (a) infrared spectrum of the sample dissolved in liquid xenon at 2758C; (b) calculated spectrum with DH value of 76 cm 21; (c) calculated spectrum with DH value of 126 cm 21. Both the calculated spectra have the trans conformer the more stable rotamer.

than observed in the spectrum of the liquid (Fig. 4). Also, there are some problems in the CH stretching region of the calculated spectra where a number of observed bands in the infrared spectrum are more intense than predicted and the calculated activities of the predicted Raman spectrum are significantly weaker than what was observed. For ethylphosphonothioic difluoride, the predicted frequencies and intensities of the infrared spectrum agree well with those obtained experimentally (Fig. 2). However, the P–C stretch is predicted too weak when compared to the Raman spectrum of the liquid. Although there are some differences in the calculated versus experimental intensities, particularly for the CH stretching region, these data demonstrate the utility of ab initio calculations in predicting spectra for conformer identification as well as for analytical purposes. From the experimental data, the ethylphosphonic difluoride molecule exhibits conformational equilibrium

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with the trans conformer more stable than the gauche form by 76 ^ 9 cm 21 (0.91 ^ 0.11 kJ/mol) whereas ethylphosphonothioic difluoride has the gauche conformer more stable in the fluid phase by 53 ^ 7 cm 21 (0.63 ^ 0.08 kJ/mol) and the trans conformer the form that predominates in the solid. The ab initio calculations support the findings for the CH3CH2P(S)F2 molecule, but the conformational stability of the CH3CH2P(O)F2 molecule is reversed in favor of the gauche form in the lower level calculations (RHF/6-31G(d) and MP2/6-31G(d)), and is not predicted correctly until the MP2/6-31111G(d,p) level is used. As the predicted energy difference of ethylphosphonic difluoride is small, the correct determination of the enthalpy difference is necessary for the calculation of the theoretical spectra (Figs. 1, 2, 4 and 5). In Fig. 10 the comparison of the experimental spectrum versus the calculated spectra are shown at two different values of DH (both having the trans conformer the more stable form) for the P–C stretching region of CH3CH2P(O)F2. It is evident that the calculated spectrum with the previously determined [4] DH value of 126 cm 21 (Fig. 10(c)) reverses the intensities of the bands, and thus does not correspond to the experimental spectrum. To better understand the conformational stabilities of these molecules, it is useful to compare their structural parameters (Tables 3 and 4). The structural parameters calculated for the two different conformers of CH3CH2P(O)F2 are very similar with major differences being the PCC, PCH, and HC1H angles. For the trans conformer, the PCC angle is approximately 2.78 larger, and the PCH and HC1H angles are 1–28 smaller than the corresponding gauche values. These differences are also obtained from the microwave data [5] (Table 3) except for the HC1H angle which was a fixed parameter. As for the structural differences between the two conformers of CH3CH2P(S)F2 (Table 4), the ab initio calculations indicate an opening of the PCC angle by 1–28 when going from the gauche to the trans conformer and smaller differences are calculated for the PCH and HC1H angels (0.5–l.18). Similar differences are reported for the experimental data [6] (Table 4) where these parameters vary by 2.5, 1.1, and 0.258 for the PCC, PCH, and HC1H angles, respectively. The structure of some small molecules containing

148

Parameters

F3PO r(PyO) r(P–F) \FPF F3PS r( PyS) r(P–F) \FPF a

MW a

ED b

RHF/631G(d)

MP2/631G(d)

MP2/6311G(d)

MP2/63111G(d)

(roadj ) c

MP2/63111G(2d)

MP2/63111G(2df)

MP2/63111G(3df)

(roadj ) d

1.437(4) 1.522(4) 101.14(10)

1.436(6) 1.524(3) 101.3(2)

1.425 1.526 100.7

1.458 1.558 100.3

1.446 1.550 100.5

1.449 1.554 100.6

1.441 1.521 e 101.3

1.445 1.539 100.6

1.442 1.533 100.8

1.438 1.531 100.8

1.431 1.524 e 100.9

1.866(5) 1.538(3) 99.6(3)

1.874 1.535 99.6

1.870 1.569 99.1

1.864 1.560 99.3

1.867 1.565 99.3

1.865 1.534 c 99.8

1.881 1.550 99.1

1.868 1.544 99.4

1.860 1.542 99.3

1.861 1.535 e 99.5

1.849(3) 1.538(10) 98.5(20)

From Ref. [19] for F3PO and Ref. [21] for F3PS. From Ref. [20] for F3PO and Ref. [22] for F3PS. c Initial parameters taken from the MP2/6-311 1 G(d) calculation. d Initial parameters taken from the MP2/6-311 1 G(3df) calculation. e Initial values from the RHF/6-31G(d) calculation. b

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

Table 10 Structural parameters for F3PO and F3PS

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

the P(O)F2 and P(S)F2 moieties have already been determined quite reliably by microwave or electron diffraction methods [19–29]. It is feasible to compare the experimentally determined structural parameters of such compounds with the calculated parameters with different basis sets and levels of ab initio calculations. This allows the selection of the most appropriate basis sets to reliably predict the geometry of the larger chemical analogues or to obtain systematic corrections for specific structural parameters. In Table 10, we have listed the geometric parameters of F3PO and F3PS as obtained from microwave [19,21] and electron diffraction [20,22] data together with the predictions from a variety of basis sets up to MP2/6311 1 G(3df). The PyO bond lengths are reasonably predicted at the MP2 level if the triple-split-valence (6-311) basis sets are used. However, the P–F distances are calculated too long at the MP2 level unless an expensive addition of diffuse orbitals (3df) is used. Actually, the RHF/6-31G(d) calculation gives a much more reasonable value for the P–F distance, and we recommend it for predictions of this parameter in P-containing compounds. The values of the FPF angles are quite good at all levels of calculation. Resulting from a limited number of possible isotopic substitutions in F3PO and F3PS and the C3v symmetry of the molecules, only the B rotational constant can be obtained reliably from the microwave data. This fact leads to a significant correlation between the structural parameters in the fit of the ro structure. It is possible, however, to incorporate the predicted differences among the structural parameters obtained from ab initio calculations in the determination of the molecular geometry assuming that the errors associated with ab initio calculations are mainly systematic. A computer program [30] developed in our laboratory consists of an iterative procedure which starts from the ab initio structures and modifies them in order to fit the observed rotational constants. The program restricts the changes of all or a selected number of equivalent geometric parameters in a way that keeps the differences among them unchanged as predicted by the ab initio calculations. In addition to the fitting of the observed rotational constants, the program also minimizes the differences between the initial (ab initio) structural parameters and their current values during the iterations. However, these

149

differences are given only a small weight in the overall fit. In that sense, the initial values of the structural parameters influence the outcome of the structural fit. For example, in Table 10 we have listed ro structures obtained by the application of the described method and the microwave rotational constants [21,27–29] using initial geometries taken from the MP2/6-311 1 G(d) and MP2/6-311 1 G(3df) calculations with the exception of the P–F distance which was taken from the RHF/6-31G(d) basis set. The final ˚ differences in the PyO results show 0.01 and 0.003 A and P–F distances, respectively. Thus, the two obtained geometries can no longer be described as pure ro structures, but rather as modified or adjusted ro structures, and to acknowledge this, we have labeled them with roadj . For the F3PS molecule the two roadj structures are consistent with each other, but different from the previously reported [21] microwave structures where the PyS distance is much shorter. Our result, however, is in agreement with the electron diffraction data, which should be more reliable for estimation of the PyS distance. Again, the importance of good initial parameters cannot be underestimated. This is especially true for larger molecules, where the experimentally determined rotational constants are less than the number of geometric parameters of the molecule. For example, the molecules CH3P(O)F2 and CH3P(S)F2 have 3 and 6 experimentally determined rotational constants, respectively [23,25,26], and 18 structural parameters. Obviously, not all geometric parameters can be allowed to change in the structural fit, and in the previous studies [23,25] most of the parameters were kept fixed at some assumed values. Using the above mentioned method and the previously reported [23,25,26] rotational constants, we obtained the roadj structures of CH3P(O)F2 and CH3P(S)F2 listed in Tables 11 and 12, respectively. The starting geometries were taken from the MP2/6311 1 G(d,p) calculations corrected for the shorter P– F distances from the RHF/6-31G(d) basis set. The heavy atom parameters are in good agreement with the previously reported [23,25] ones with the exception of the P–C bond distances which are 0.025 and ˚ shorter for CH3P(O)F2 and CH3P(S)F2, 0.013 A respectively, in this study. However, with the help of electron diffraction data [24], it has been shown

150

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Table 11 Structural parameters for CH3P(O)F2 Parameters

MW a (ro)

MW1Ed b (rz)

RHF/6-31G(d)

MP2/6-31G(d)

MP2/6-3111G(d,p)

This study (roadj )

r(P–C) r(PyO) r(P–F) \OPC \OPF \FPC \FPF

1.795(19) 1.442(9) 1.544(11) 118.2(15) 117.5(20) 100.5(23) 99.2(10)

1.770(5) 1.444(3) 1.545(2) 117.8(8) 115.0(8) 103.7(8) 99.2(2)

1.786 1.437 1.548 118.4 114.9 103.6 98.8

1.784 1.470 1.581 118.7 115.4 103.0 98.6

1.779 1.461 1.579 119.1 114.9 103.2 98.8

1.770 1.456 1.545 119.2 115.3 102.7 99.1

a b

From Ref. [23]. From Ref. [24].

Table 12 Structural parameters for CH3P(S)F2 Parameters

MW a (ro)

RHF/6-31G(d)

MP2/6-31G(d)

MP2/6-3111G(d,p)

This study …roadj †

r(P–C) r(PyS) r(P–F) \SPC \SPF \FPC \FPF

1.799(9) 1.877(4) 1.550(2) 119.1(9) 116.2(8) 101.9(4) 98.5(2)

1.794 1.897 1.556 120.0 115.4 102.3 98.3

1.790 1.890 1.591 120.0 116.1 101.6 97.9

1.786 1.886 1.590 120.6 115.6 101.7 98.1

1.786 1.880 1.555 119.9 116.0 102.2 98.1

a

From Ref. [25].

Table 13 Rotational constants (MHz) of some RP(O)F2 and RP(S)F2 molecules (R ˆ F and CH3) Molecule

Rotational constants

Obs. a

Calc. b

Obs.2 calc.

Ref.

F3P 16O

A B A B B B B B B A B C A B C A B C

4811.756 4594.263 4811.758 4594.262 4395.287 2657.653 2617.757 2617.757 2579.790 4495.91 4271.86 4125.90 4366.24 2596.38 2496.86 4366.02 2522.84 2428.65

4811.663 4593.454 4811.663 4593.454 4394.295 2657.763 2617.874 2617.874 2579.901 4495.68 4271.65 4123.64 4366.58 2595.84 2496.81 4366.21 2522.30 2728.59

0.093 0.809 0.091 0.808 0.992 0.110 0.123 0.123 0.111 0.23 0.21 2.26 0.34 0.54 0.05 0.19 0.54 0.06

[27]

F3P 16O F3P 18O F3P 32S F3P 32S F3P 33S F3P 34S CH3P(O)F2

CH3P( 32S)F2

CH3P( 34S)F2

a b

From microwave data. From the roadj structures obtained in this study (Tables 10–12).

[19] [28] [29] [21] [21] [21] [23]

[26]

[25]

J.R. Durig et al. / Journal of Molecular Structure 516 (2000) 131–152

151

Table 14 Rotational constants (MHz) of some CH3CH2P(O)F2 and CH3CH2P(S)F2 Molecule

12

CH3 12CH2P 32SF2

12

CD3 12CD2P 32SF2

12

CD3 12CH2P 32SF2

12

CH3 12CH2P 34SF2

12

CD3 12CD2P 34SF2

13

CH3 12CH2P 32SF2

12

CH3 13CH2P 32SF2

13

CH3 12CH2P 34SF2

12

CH3 13CH2P 34SF2

CH3CH2P(O)F2

Rotational constants

A B C A B C A B C A B C A B C A B C A B C A B C A B C A B C

Trans

Gauche

Obs. a

Calc. b

Obs.2 cal.

Obs. a

Calc. b

Obs.2 cal.

3873.71 1664.09 1564.37 3578.82 1510.26 1409.92 3779.09 1526.22 1441.36 3860.10 1624.81 1527.39 3565.00 1475.75 1377.69 3865.15 1628.24 1531.33 3834.16 1656.54 1551.45 3850.82 1590.00 1495.23 3821.71 1617.38 1514.81 4335.81 c 2343.936 c 2287.766 c

3873.54 1664.12 1564.38 3578.80 1510.32 1409.92 3779.12 1526.24 1441.31 3859.71 1624.75 1527.38 3565.01 1475.72 1377.67 3865.00 1628.30 1531.35 3835.36 1656.55 1551.48 3850.60 1589.98 1495.24 3821.46 1617.33 1514.84 4335.867 2344.904 2288.638

0.17 0.03 0.01 0.02 0.06 0.00 0.03 0.02 0.05 0.39 0.06 0.01 0.01 0.03 0.02 0.15 0.06 0.02 1.20 0.01 0.03 0.22 0.02 0.01 0.25 0.05 0.03 0.057 0.968 0.872

2711.62 2117.64 1625.27 2565.33 1874.66 1472.77 2633.10 1934.78 1502.54 2669.27 2088.23 1592.82 2520.58 1852.99 1444.74 2699.43 2069.87 1592.71 2705.08 2093.79 1613.38 2654.25 2043.14 1561.28 2663.03 2064.34 1581.09 4277.596 c 2292.464 c 2251.988 c

2711.65 2117.80 1625.24 2565.25 1874.68 1472.78 2633.26 1934.73 1502.61 2669.34 2088.24 1592.82 2520.50 1852.94 1444.77 2699.48 2070.06 1592.76 2704.85 2093.64 1613.25 2654.39 2043.23 1561.35 2662.91 2064.13 1580.99 4276.830 2244.102 2249.360

0.03 0.11 0.03 0.08 0.02 0.01 0.16 0.05 0.07 0.07 0.01 0.00 0.08 0.05 0.03 0.05 0.19 0.05 0.23 0.15 0.13 0.14 0.09 0.07 0.12 0.21 0.10 0.766 1.638 2.628

a

From Ref. [6]. From the roadj structures obtained in this study (Tables 3 and 4). c From Ref. [5]. b

that the P–C distance in CH3P(O)F2 should be about ˚ which is consistent with our roadj 1.770 ^ 0.005 A value for this parameter (Table 11). The fitting of the rotational constants for all molecules mentioned above are given in Table 13, and are acceptable in all cases. The data given in Tables 10–13 demonstrate that reasonably good roadj structures can be obtained with the help of ab initio predictions even when the amount of experimental data is limited. The MP2/6311G1(d,p) calculations with a single correction for the length of the P–F bond distance from the RHF/631G(d) basis set give excellent initial values for the

structural fit. Therefore, we applied the described method to determine more complete roadj structures for the two conformers of CH3CH2P(O)F2 and CH3CH2P(S)F2 than the previously reported [5,6] ro parameters. As only three rotational constants were available for each conformer of the CH3CH2P(O)F2 compound, we were able to vary only 10 of the 27 parameters of the molecule (Table 3). The rest of the parameters (mainly carbon–hydrogen distances and angles) were kept fixed at the ab initio (MP2/6311 1 G(d,p)) values, which were used as starting geometries. The abundance of experimental data for the different isotopomers of CH3CH2P(S)F2 molecule

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allowed the elucidation of all 27 geometric parameters of both conformers (Table 4). The major disagreement between the results from this study and the previously reported [5] ones (Table 3) is in the P–C distances of the CH3CH2P(O)F2 ˚ shorter. Addicompound which now are about 0.02 A tionally, two of the fixed parameters, the C–C and ˚ longer in the current PyO distances, are about 0.01 A results compared to the previously reported values. However, for the CH3CH2P(S)F2 compound the agreement is much better (Table 4), because the large number of rotational constants make the initial structures less important in the fit. Nevertheless, the P–C ˚ longer distance of the trans conformer is still 0.016 A in the previous study [6], whereas the PyS distance is ˚ longer in the current results. These last two 0.016 A discrepancies are not seen in the geometry of the gauche conformer, but it should be noticed that all utilized basis sets of ab initio calculations predict nearly identical values for the P–C and the PyS distances in the two conformers and this is consistent with our results. Satisfactory fits of the rotational constants were achieved for both molecules, and they are shown in Table 14. These latter results clearly show that reliable structural parameters can be obtained for these types of phosphorus molecules by a combination of ab initio results and a limited number of rotational constants from microwave studies. Acknowledgements JRD would like to acknowledge partial support of these studies by the University of Missouri-Kansas City Faculty Research Grant program. References [1] J.R. Durig, J.B. Robb II, J. Mol. Struct. 413 (1997) 371. [2] J.R. Durig, J.S. Church, C.M. Whang, R.D. Johnson, B.J. Streusand, J. Phys. Chem. 91 (1987) 2769. [3] P. Groner, J.S. Church, Y.S. Li, J.R. Durig, J. Chem. Phys. 82 (1985) 3894.

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