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Thiophenol and thiophenol radical and their complexes with gold clusters Au5 and Au6
Journal of Molecular Structure 708 (2004) 165–173 www.elsevier.com/locate/molstruc
Thiophenol and thiophenol radical and their complexes with gold clusters Au5 and Au6 F. Remaclea,*, E.S. Kryachkoa,b,1 a
Department of Chemistry, Bat. B6c, University of Liege Sart-Tilman, B-4000 Liege 1, Belgium b Bogoliubov Institute for Theoretical Physics, Kiev 03143 Ukraine Received 20 February 2004; revised 22 February 2004; accepted 22 February 2004 Dedicated to Prof. Galyna Puchkovskaya Available online 28 July 2004
Abstract The longstanding controversy between experiment and theory regarding which conformer of thiophenol, planar or perpendicular, is the most stable and what is the magnitude of the corresponding rotational barrier of the S– H group is discussed. We propose a variety of rather modest high-level computational methods within the density theory, which corroborate the experimental data. These methods demonstrate that the planar structure of thiophenol is the most stable and the magnitude of the rotational barrier falls within the experimental range of 3.35 ^ 0.84 kJ mol21. However, the barrier is of the order of RT at room temperature, which might prevent to clearly identify the most stable conformer of thiophenol in experiments and leads to a large-amplitude motion of the thiolic hydrogen. On the other hand, such low value of the barrier may lead to some error in evaluating the thermodynamic properties of thiophenol within the rigid-rotor-harmonic oscillator model, in particular for the bond dissociation enthalpy. We also show the existence of a large entropy contribution to the Gibbs free energy difference between the planar and perpendicular conformers which is the order of the rotational barrier (<4 kJ mol21). This might be of interest for experimental study. The most stable complexes of thiophenol with the gold clusters Au5 and Au6 are also investigated. It is shown that the sulfur atom prefers to anchor to two- and three-coordinated atoms of gold in these clusters to form a strongly directional gold – sulfur bond. The hydrogen abstraction from the S– H group of thiophenol bonded to the two-coordinated gold atom in Au5 yields the bridging Au –S dibond and results in a spectacular reduction of the bond dissociation energy of thiophenol by nearly a factor of three. q 2004 Elsevier B.V. All rights reserved. Keywords: Thiophenol; S–H bond; Rotational barrier; Thiophenol radical; Bond dissociation energy; Gold–sulfur bond; S– H cleavage
1. Introduction: thiophenol Thiophenol (benzenethiol) ArSH as a derivative of phenol where the oxygen atom is substituted by the sulfur belongs to the sulfur-containing compounds whose importance in atmospheric, environmental, biological, and medical areas can hardly be overestimated (see Refs. [1 – 4] for recent works and references therein). Moreover, the recent huge activity in nanoscience and nanotechnology has renewed the interest in these compounds, particularly in thiophenols, because of the ability of sulfur to bind gold clusters (see Refs. [6 – 18] and references therein). The first studies of thiophenol go back to 1956 when Scott et al. [19] * Corresponding author. Tel.: þ 32-4-366-2341; fax: þ32-4-366-3413. E-mail address: [email protected] (F. Remacle). 1 Co. Corresponding author. [email protected] (E.S. Kryachko). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.02.056
(see also Ref. [20]) investigated its thermodynamic properties and obtained a value of 3.35 ^ 0.84 kJ mol21 for the barrier height Vrot of the internal rotation of the S –H group from the stable planar to the transition-state perpendicular conformer. In 1974, Larsen and Nicolaisen [21] (see also Refs. [22,23]) reported for the first time the experimental microwave and far-infrared spectra of thiophenol and estimated Vrot at 3.2 kJ mol21 (3.4 kJ mol21 in the NMR study in CCl4 solution [24 – 26]; see also Refs. [27 –29]). What matters and is relevant for the present work is that these studies have concluded that the thiophenol molecule exhibits a planar structure. Note, however, that the reported experimental barrier is small and becomes of the order of RT at room temperature. This could prevent to clearly identify the most stable conformer of thiophenol. Strictly speaking, the experimental structure of thiophenol has not been determined so far. According to
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the microwave experiments by Johansson et al. [22] carried out in 1967, thiophenol is a planar molecule. This planarity has been confirmed by the early lower-level quantum chemical calculations [23,30 –32] that estimated a rotational barrier between 13.9 and 16.7 kJ mol21. On the other hand, in 1979 Lunazzi et al. [27] suggested that thiophenol is nonplanar and characterized by a twist angle of 26 ^ 38. They argued that the low rotational barrier does not prevent large-amplitude motion of the thiolic hydrogen atom far from its equilibrium position, with the mean value of the twist angle reaching 30– 408 (see also Refs. [33,34]). The 20 years following 1974 were marked by a puzzling disagreement between experiment and high level ab initio theory [35 – 37]. For instance, a rather high MP2/ 6-311G(d,p)//HF/6-311G(d,p)(5D) computational method favored a perpendicular orientation of the S – H group relative to the phenyl ring over the planar by 1.95 kJ mol21 [37]. During this period the question of whether a minimum computational level there exists, say within the density functional theory (DFT), that accurately corroborates the experimental data on the planar conformer being the most stable one and on the magnitude of Vrot was regrettably left unanswered. Moreover, these issues have almost been forgotten in the last decade, except the work by Wiberg and Rablen [38] who demonstrated in 1998 that the MP2/ 6-311þ þ G(d,p) favors the perpendicular form ArSH’ and estimates the rotational barrier as equal to 2.1 kJ mol21. Nowadays, numerous computational works on a variety of thiophenol properties are conducted worldwide. They in particular focus on evaluation of its so called homolytic bond dissociation energy as one of key tool in determining antioxidant activity [39 – 41] (BDE; see below for the definition; see also Ref. [4] for current review) and on the formation of the gold – sulfur bond (Refs. [9,11 –14,16] and references therein) resulting in the S– H group cleavage. The latter still remains a controversial issue (see Refs. [5 – 8, 10,15,18,42] and references therein). It thus seems worth to finally resolve the aforementioned discrepancy between experiment and theory and to show which consequences can be then drawn. This is the goal undertaken in the present work.
keyword and the ‘Ultrafine’ integration. Single-point (sp) calculations were also carried at the B3LYP/ 6-311þ þ G(2df,2p) (; D)//B3LYP/A level. The harmonic vibrational frequencies and corresponding unscaled zero-point vibrational energies (ZPVE) were calculated at all employed DF/X levels (X ¼ A, B, and C) in order to distinguish the topology of the stationary points and to obtain the thermodynamic properties of the studied complexes. All reported relative energies include the ZPVE correction. Many computational studies on the thiol-gold clusters [46] suggested the basis set superposition error to be negligible (less than 0.015 eV) on the relevant accuracy scale related particularly with the usage of the RECPs and is therefore not taken into account in the present work.
3. Thiophenol: geometries, Vrot and BDE Some selected properties of thiophenol in its planar (Fig. 1) and perpendicular conformers necessary for a further discussion are gathered in Table 1. We first note that the B3LYP/A geometry of the planar conformer is almost equivalent to that optimized at the higher B3LYP/ 6-311þ þ G(d,p) level in Ref. [47] and that the C1 – S7 ˚ at the MP2/6-31 þ G(d) [38] bond length of 1.7753 A satisfactorily matches the present value. The agreement with the experimental microwave geometry [22]: rexpt(C1 –S7) ¼ ˚ and /exptC1S7H7 ¼ 998 is ˚ , rexpt(S7 – H7) ¼ 1.30 A 1.77 A also rather fair. Interestingly, the early measured rotational constants A ¼ 5588:0; B ¼ 1577:7; and C ¼ 1231:1 MHz [22] do not allow to distinguish between the two conformers because, as follows from Table 1, their computed values are very close: DA ¼ 9 – 11 MHz, DB ¼ 18 – 20 MHz, and DC ¼ 1 – 2 MHz.
2. Computational framework All calculations of thiophenol and thiophenol (thiyl) radical and their complexes with gold clusters, conducted in the present work, were performed with the density functionals (DFs) B3LYP, MPW1K [43], BLYP in conjunction with a variety basis sets 6-311G(d,p) (; A), 6-311 þ G(2d,2p) (; B), and cc-pVDZ (; C) (for ArSH and ArSz) and one of the mostly employed energy-consistent 19-electron relativistic effective core potential (RECP) developed by Ermler, Christiansen and co-workers [44] using GAUSSIAN 03 package of quantum chemical programs [45]. These optimizations were conducted with the ‘Tight’
Fig. 1. View of the thiophenol molecule and its relevant atomic numbering.
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Table 1 ˚ ), bond angles (in 8), rotational constants A; B; and C (in MHz), total dipole moment (in D), S –H stretching frequency (in cm21 followed The bond lengths (in A by the IR activity in km mol21 in parentheses), imaginary transition frequency nts (in ı cm21, only for the ArSH’ conformer), electronic energy E (in hartree), ZPVE (in kcal mol21), enthalpy H (in hartree) and entropy S (in cal mol21T21) of the thiophenol ArSH in the stable planar ArSH and transition-state perpendicular ArSH’ conformers and thiophenoxyl radical ArS· B3LYP/A
B3LYP/B
B3LYP/C
MPW1K/A
MPW1K/B
MPW1K/C
BLYP/A
BLYP/B
BLYP/C
ArSH r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C4 –C5) r(C5 –C6) r(C1 –C6) r(C1 –S7) r(S7 –H7) /C1 S7 H7 A B C D nCS nCC a nSH E þ 630 Esp þ 630 ZPVE þ 60 H þ 630 S þ 80
1.398 1.392 1.393 1.393 1.391 1.398 1.789 1.347 96.8 5613.2727 1564.6523 1223.5876 1.23 409 (0.5) 1112 (30) 2680 (2) 20.521611 20.547056 1.98 20.416396 1.56
1.395 1.390 1.391 1.391 1.389 1.396 1.784 1.340 97.0 5633.8077 1570.7138 1228.2702 1.08 409 (0.5) 1113 (31) 2674 (0.5) 20.538067
1.403 1.397 1.397 1.398 1.396 1.404 1.790 1.357 96.8 5569.8338 1556.0929 1216.2879 1.05 410 (0.5) 1115 (30) 2668 (2) 20.470375
1.402 1.396 1.397 1.397 1.395 1.403 1.783 1.350 96.5 5579.8115 1563.9461 1221.5594 1.00 401 (0.5) 1086 (33) 2610 (0.4) 20.504904
1.409 1.402 1.403 1.404 1.401 1.411 1.790 1.365 96.8 5520.2724 1549.6171 1209.9635 1.00 401 (0.5) 1089 (32) 2613 (2) 20.440132
1.414 1.407 1.408 1.408 1.406 1.415 1.807 1.369 96.8 5488.0586 1531.9578 1197.6436 0.95 395 (0.8) 1073 (26) 2568 (4) 20.336954
2.11 20.365058 0.13
0.65 20.401831 0.04
0.67 20.337012 0.14
1.408 1.402 1.403 1.403 1.401 1.409 1.807 1.358 96.5 5531.5735 1539.5370 1204.3458 1.16 393 (0.9) 1066 (31) 2583 (4) 20.396557 20.421127 0.11 20.294203 1.35
1.406 1.400 1.401 1.402 1.399 1.407 1.800 1.351 97.0 5550.7087 1545.9646 1209.1862 1.01 394 (0.8) 1068 (28) 2578 (1) 20.412466
2.20 20.432609 0.10
1.404 1.398 1.398 1.399 1.396 1.405 1.788 1.356 96.3 5562.1472 1557.7796 1216.9506 1.15 401 (0.6) 1086 (31) 2620 (2) 20.488924 20.512918 0.54 20.385951 0.66
0.34 20.309837 0.36
0.24 20.234484 0.36
ArSH’ r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C1 –S7) r(S7 –H7) /C1S7H7 /C4C2S7C6 A B C D nCS nCC a nSH nts E þ 630 Esp þ 630 ZPVE þ 60 H þ 630 S þ 76 DEðArSH’ – ArSHÞ DHðArSH’ – ArSHÞ DG298 K ðArSH’ – ArSHÞ
1.397 1.393 1.393 1.809 1.351 97.1 1.0 5602.7115 1546.3636 1222.1014 1.8 408 (0.2) 1106 (4) 2638 (7) 20 20.521297 20.545462 1.93 20.417003 0.61 0.83 21.6 4.6
1.394 1.391 1.391 1.804 1.344 97.1 1.0 5623.6884 1551.1467 1225.9722 1.7 408 (0.3) 1107 (3) 2638 (2) 118 20.536800
1.402 1.398 1.398 1.811 1.360 96.8 1.1 5558.7332 1537.2430 1214.4645 1.6 407 (0.2) 1107 (3) 2629 (2) 112 20.469064
1.400 1.397 1.397 1.806 1.353 96.6 1.1 5568.4547 1542.9358 1218.3807 1.7 398 (0.2) 1078 (3) 2578 (2) 148 20.502706
1.407 1.404 1.404 1.812 1.368 96.4 1.1 5510.5249 1529.7390 1207.6133 1.6 398 (0.1) 1079 (3) 2575 (2) 140 20.437995
1.412 1.408 1.408 1.829 1.372 97.1 1.0 5479.4434 1512.0027 1195.1229 1.5 391 (0.1) 1063 (4) 2528 (12) 139 20.335016
1.97 20.364714 0.64 3.44 (2.85) 0.9 5.3
0.47 20.400596 1.13 5.77 (5.04) 3.2 6.9
0.48 20.335865 1.18 5.61 (4.81) 3.0 6.7
1.406 1.403 1.403 1.828 1.362 96.9 1.2 5522.6358 1520.5102 1202.3200 1.8 389 (0.1) 1062 (5) 2541 (11) 88 20.395719 20.419063 0.03 20.294292 1.25 2.20 (1.87) 20.2 4.9
1.405 1.401 1.401 1.822 1.355 97.1 1.0 5542.5770 1525.7589 1206.4287 1.7 391 (0.2) 1063 (4) 2542 (4) 140 20.410664
2.09 20.432269 0.56 3.33 (2.86) 0.9 5.3
1.402 1.399 1.399 1.810 1.359 96.5 1.2 5551.6844 1538.1954 1214.7166 1.8 397 (0.2) 1077 (4) 2583 (7) 97 20.487725 20.510431 0.41 20.385694 1.17 3.14 (2.63) 0.7 5.0
0.20 20.308984 1.18 4.73 (4.11) 2.2 6.2
0.07 20.233526 1.23 5.09 (4.36) 2.5 6.4
ArSz r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C1 –S7) E þ 629 Esp þ 629 ZPVE þ 54 H þ 629
1.401 1.392 1.393 1.771 20.876612 20.901068 2.56 20.780869
1.398 1.389 1.391 1.767 20.891987
1.422 1.391 1.403 1.733 20.842738
1.422 1.388 1.402 1.725 20.874001
1.429 1.395 1.409 1.732 20.812801
1.434 1.400 1.414 1.744 20.717154
2.72 20.746782
1.33 20.780099
1.37 20.718849
1.430 1.394 1.409 1.740 20.775496 20.797242 0.95 20.682185
1.427 1.392 1.407 1.737 20.788847
2.77 20.795944
1.425 1.390 1.405 1.728 20.860544 20.881794 1.24 20.766774
1.14 1.04 20.695266 20.623719 (continued on next page)
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Table 1 (continued)
S þ 77 DEðArSH – ArSz Þb DEðArSH – ArSz Þb,c DHðArSH – ArSz Þb
B3LYP/A
B3LYP/B
B3LYP/C
MPW1K/A
MPW1K/B
MPW1K/C
BLYP/A
BLYP/B
BLYP/C
0.96 358.0 360.6 333.2
0.89 360.8
0.87 312.6
1.45 321.5
1.45 312.1
1.46 292.8
288.0
297.4
288.1
1.53 296.3 303.7 272.5
1.43 302.8
336.1
1.51 314.9 322.1 290.7
279.0
269.0
21
The energy, enthalpy and Gibbs differencies are given in kJ mol . The numbering of atoms is indicated in Fig. 1. The superscript sp corresponds to the DF/D//DF/A computational level after the DF/A unscaled ZPVE. a nCC was incorrectly assigned to nCS ¼ 1120 cm21 in Ref. [59]. In fact, this is the combinational mode of the phenyl ring composed of nCC ; dCH ; and dCCC (see Refs. [4,65] and references therein). b The exact energy of 20.5 hartree of the ground-state hydrogen atom is used to avoid the well-known self-energy problem of the employed DFT potentials (see also Refs. [53,58]). c The BDE is evaluated at the DF/D//DF/A computational level taking the DF/A unscaled ZPVE into account.
The main difference between the geometries of the two conformers of thiophenol is the length of the C1 –S7 bond that elongates under the planar ) perpendicular transition ˚ . The S 7 – H 7 bond stretches by only by < 0.02A ˚ that redshifts its stretching vibrational 0.001 – 0.004 A mode nSH by about 40 cm21 (Table 1). Note that the nSH stretch was experimentally observed in the liquid phase at 2584 cm21 [19]. It is worth mentioning a small nonplanarity of the perpendicular form manifested in that the C1 – S7 bond lies out of the phenyl ring by ca. 18 (Table 1), although it might be attributed to a deficiency of all employed computational levels. Some discrepancies in the geometries of both forms are related to the employed computational level (without any reference to the undetermined experimental geometry): (i) BLYP systematically overestimates the bond lengths compared to B3LYP and MPW1K. For C –C bonds, this discrepancy falls within a ˚ . This margin doubles for the narrow margin of 0.01 A C1 – S7 bond. (ii) Within a given density functional, there arises a sort of basis discrepancy. It means that for both aforementioned bonds, the basis A systematically ˚ compared overestimates their lengths by 0.001– 0.005 A to B and underestimates them within nearly the same margin in comparison to C. These issues could only be resolved by a straightforward comparison with an experimentally determined geometry. Interestingly, all employed computational methods favor the planar conformer over the perpendicular although their energetic offset Vrot varies from method to method (Table 1). If, for instance, B3LYP/A estimates it as 0.8 kJ mol21 (but B3LYP/D//B3LYP/A gives 4.2 kJ mol21), B3LYP/B and B3LYP/C fairly agree with the experimental magnitude expt ¼ 3:35 ^ 0:84 kJ mol21 of the rotational barrier Vrot [19,20]. The MPW1K in conjunction with the basis A falls into the same range of Vrot whereas its usage with the bases B and C (including the single-point MPW1K/D// MPW1K/A) overestimates the barrier height. The BLYP with the bases B and C (also together with BLYP/ D//BLYP/A) shows a similar tendency although with A, on the contrary, it underestimates Vrot : The potential energy function of the internal rotation of the S7 – H7 group by angle u around the C1 –S7 bond
can be rather accurately approximated by VðuÞ ¼ Vrot sin2 u [21,33,36]. Assuming that this internal rotation of the S7 – H7 group is well separated from the other vibrational modes of thiophenol, one can write the corresponding Schro¨dinger equation "2 00 c þ ðEn 2 Vrot sin2 uÞcn ¼ 0 2I n
ð1Þ
for the nth-state wavefunction cn ðuÞ characterized by the eigenenergy En : In Eq. (1), I is the corresponding moment of inertia, cn 00 denotes the second derivative of cn with respect to u: Eq. (1) is the Mathieu equation [48],
cn 00 þ ½1n þ 2q cosð2uÞcn ¼ 0
ð2Þ
where
2I 1 V E 2 n 2 rot "2 I 2q ¼ 2 Vrot : " 1n ¼
According to Table 2 of Ref. [21], 2q ¼ 13:688 so one can use the approximate formula [49] for the eigenvalues 1n : pffiffi 1n < 22q þ 2ð2n þ 1Þ q ð3Þ that in turn determines the equidistant spectrum 1 Vrot En ¼ n þ pffiffi : 2 q
ð4Þ
expt Substituting Vrot ¼ 3:35 ^ 0:84 kJ mol21 ¼ 280 ^ 70 cm21 and 2q ¼ 13:688 into Eq. (4), one obtains that E0 ¼ 53:5 ^ 13:5 cm21 and E1 ¼ 160:5 ^ 40:5 cm21. The difference E1 2 E0 determines the torsional frequency nt ¼ 107 ^ 27 cm21 of the S7 –H7 group. The obtained value of nt is in a fairly good agreement with the range of 93 –128 cm21 calculated at all employed computational levels, except the B3LYP/A and BLYP/A, which underestimate nt giving 46 and 74 cm21, respectively, and 21 the observed nexpt (see Fig. 1 and Table 3 of t ¼ 90:9 cm Ref. [21]). The computed entropies of the planar and perpendicular forms of thiophenol are given in Table 1. Their difference is quite large, between 3 and 5 cal mol21 T21 which gives rise
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to a large entropy contribution (about 4 kJ mol21) to the Gibbs free energy at room temperature (Table 1). The latter expt is comparable with Vrot and hence such large entropy effect might be observable experimentally. The fact that the experimental magnitude of the rotational barrier is of the order of RT at T ¼ 298 K, viz., 3.2 vs. 2.5 kJ mol21, may lead to some error in evaluating the thermodynamic quantities within the rigid-rotor-harmonic-oscillator (RRHO) model [50,51], which is implemented in the GAUSSIAN 03 suit of programs [45]. This may further result in an insufficient accuracy for estimating the so-called bond dissociation enthalpy of thiophenol and comparing it with the experimental one. Let us nevertheless neglect this error since it deserves a separate study and analyze the values of the bond dissociation energy of the S – H group of thiophenol that we compute. The simplest definition of the bond dissociation energy of the S – H bond of thiophenol (BDEthiophenol(S – H) for shorthand; for the complete definition of the bond dissociation energy and enthalpy see Refs. [4,39 – 41]) is the ZPVE-corrected energy difference between the ground state of ArSH and that of the radical ArSz. Some relevant properties of the latter are gathered in Table 1. One observes that the abstraction of H7 strengthens the C1 –S7 bond due to a stronger conjugation effect that increases the double-bond character. This in turn slightly weakens the neighbouring C1 – C2 and C1 –C6 bonds. The difference in the energies and enthalpies of ArSH and ArSz (last two lines of Table 1), both computed with the unscaled ZPVE, show a strong dependence on the computational level. The differences between the different levels fall within a range of < 70 kJ mol21. The upper-bound estimate to the BDEthiophenol(S– H), equal to 360.8 kJ mol21, is obtained within the B3LYP/B method whereas the lower-bound of 292.8 kJ mol21 is given by BLYP/C. The MPW1K DF potential provides a value of BDEthiophenol(S –H) within a range of 312.1 – 321.5 kJ mol21 while BDE amounts to 322.1 kJ mol21 at MPW1K/D// MPW1K/A after the MPW1K/A ZPVE. The corresponding enthalpy difference ranges from 272.5 kJ mol21 (BLYP/A) to 336.1 kJ mol21 (B3LYP/B). It completely falls within the wide range of about 16 kJ mol21 (precisely, after some correction, from 331.0 to 349.4 kJ mol21 [52]) obtained for the available experimental values [53 –57] of this quantity and its early reported theoretical estimates [54,58,59] (see also Table 1 in Ref. [58]).
4. Complexes of thiophenol and thiophenyl radical with gold clusters Au5 and Au6 In this Section, we discuss ArSH –Au5#n # 6 complexes as prototypical model systems to study the gold –sulfur bond between arylthiol molecules and gold clusters. The latter are chosen in their planar electronic ground states [60 – 62]. The potential energy surface (PES) of ArSH – Au5 presents three stable conformers and their rotamers. Fig. 2
169
shows that the conformer I 5 and its rotamer II 5 , distinguished from I5 by the orientation of phenyl ring relative to the plane of the gold cluster, and other two conformers III5 and IV5. There also exist one rotamer of III5 and three rotamers of IV5 (not shown in Fig. 2). I5 is slightly more stable by 0.7 kJ mol21 than II5 (Table 2). I5 and III5 have practically the same electronic energy. IV5 has an energy 28.3 kJ mol21 higher than I5. All mentioned structures are highly polar (7.4, 7.0, 8.1 and 5.6 D). In I5 and II5, the sulfur atom is anchored to threecoordinated atom Au1 (Fig. 2) and forms the covalent ˚ long. Compared to the bare cluster, Au1 – S7 bond of 2.47 A the gold cluster Au5 in I5 and II5 undergoes quite drastic geometrical changes, in contrast to the thiophenol molecule where major ones occur at its C1 – S7 and S – H bonds. The ˚ while the latter by former is elongated by 0.024 –0.025 A ˚ only 0.004 A. This is accompanied by blue shift (4 – 5 cm21) of the nCS stretch and by a red shift of 42 –46 cm21 predicted for nSH : Note that the former is found at 404 – 405 cm21 which correlates with 420 cm21 observed in a monolayer of thiophenol on silver [63] (see also Ref. [64]). The most remarkable change taking place in thiophenol under the formation of the gold –sulfur bond affects the S – H bending: the S– H bond is rotated by the dihedral angle /H7S7C3C5 ¼ 72.08 in I5 and 72.18 in II5, respectively. The Au –S bond favors the quasi-perpendicular conformer of thiophenol and shows a strong directionality: the angle /C1S7Au1 varies within 110.8 –111.18. A similar interval of the Au – S –C bond angle in the CH3S– Aun has recently been found in Ref. [45]. The gold –sulfur bond in I5 and II5 is characterized by the stretching vibrational mode nAuS centered at 316– 317 cm21. The binding energies of I5 and III5 amount to 54.2 kJ mol21 despite the fact that in the latter, the Au – S bond is slightly longer although herein the S –H group is anchored to the two-coordinated atom of gold. Let us now consider the lower-energy portion of the PES of ArSH – Au6 comprising of two conformers I6 and II6 displayed in Fig. 3. As expected, since the sulfur atom is anchored to the two-coordinated gold atom in I5 and I6, they are characterized by similar geometrical patterns in the vicinity of the gold – sulfur bond (compare Figs. 2 and 3). Furthermore, the latter has a rather similar binding energy, viz., 51.5 kJ mol21. A covalent Au – S bonding of sulfur to the four-coordinated gold atom yields the complex II6 which, by analogy with IV5, is less stable by 32.8 kJ mol21 compared to I6. The radical structures I5 and Iz6 resulting from the cleavage of the S – H bond in their parent complexes I5 and I6 are displayed in Figs. 2 and 3 together with their geometrical parameters (see also Table 2). The most interesting is the Iz5 radical where the sulfur atom is anchored to the three-coordinated Au bridge. The formation of such bridging Au –S dibond between the thiophenol radical ArSz and the gold cluster Au5 significantly (by factor
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˚ and bond angles in 8. Fig. 2. Two stable complexes of thiophenol and thiophenyl radical with cluster of five gold atoms. The bond lengths are given in A The optimized bond lengths in the W-shape cluster Au5 are the following: rðAu1 – Au2 Þ ¼ 2:740; r(Au1 – Au4) ¼ 2.831, r(Au1 – Au5) ¼ 2.722, ˚ [62]. r(Au3 –Au4) ¼ 2.678 A
of three) reduces the BDE(S – H), which is equal to 113.2 kJ mol21. This is interesting effect, reported in the present work for the first time, is due to the unexpectedly strong influence of the bridging Au – S dibond on BDEthiophenol(S– H). It might help to resolve
the longstanding controversy on to the adsorption of the thiol S – H group on gold. One the one hand, IR and Raman spectroscopy [6,7,15] have shown that the S –H stretching mode of thiols adsorbed on gold is absent. This is believed to be due to the dissociation of the S – H bond and
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Table 2 The electronic energy E (in hartree), ZPVE (in kcal mol21), enthalpy H (in hartree), entropy S (in cal mol21 T21) and relevant spectroscopic features (for notation see the legend to Table 1) of the ArSH–Au5#n # 6 and ArSz –Au5#n # 6 calculated in the present work
E ZPVE H S nSAu nCS nSH
I5
II5
III5
IV5
Iz5
I6
II6
Iz6
21310.467188 64.23 21310.347088 167.78 316 404 2638 (5)
21310.466900 64.20 21310.346836 167.41 317 405 2634 (3)
21310.467180 64.16 21310.347132 171.26 313 405 2632 (4)
21310.457100 64.07 21310.337071 170.55 121 404 2663 (5)
21309.915542 58.86 21309.804934 156.10 281, 352 416
21446.499154 64.51 21446.376247 182.17 312 405 2633 (4)
21446.486268 64.27 21446.363542 180.80 108 405 2670 (4)
21445.882438 58.78 21445.769127 180.40 350 416
an adsorption of hydrogen [53]. On the other hand, vibrational and X-ray photoelectron spectroscopy studies have demonstrated that, e.g. CH3SH on Au(111) does not dissociate via a S –H bond cleavage [5]. We suggest here that the S – H bond cleavage strongly depends on the adsorption site where thiophenol (or generally speaking, thiol) is attached to gold. If it mimics the I5-type complex, which significantly reduces the BDEthiophenol(S –H) so that it becomes comparable with the formation energy of the bridging Au – S dibond, the S –H bond is likely to dissociate and H to adsorb on gold. Clearly, this process is also determined by the shape of the transition barrier. In other cases, like for example for Iz6 where the BDE(S– H) ¼ 306 kJ mol21 is still large, the S –H bond is not broken unless the thiophenol migrates to another site.
5. Summary In the present work we have demonstrated that the longstanding controversy between experiment and theory on the relative stability of the planar or perpendicular conformers of thiophenol can be resolved using the minimum basis sets A –D within the density functional approach. All employed density functional computational methods predict that the most stable conformer is planar and the computed value of the rotational barrier Vrot is in agreement with experimental data. It is usually assumed that the DF methods, particularly the B3LYP and MPW1K employed in the present work in conjunction with rather large basis sets such as B, C and D, are more reliable than the MP2 method. However, some conformational problems
˚ and bond angles in 8. The optimized bond lengths Fig. 3. Two stable complexes of thiophenol and thiophenyl radical with Au6. The bond lengths are given in A ˚ [62]. in the cluster Au6 are the following: r(Au1 –Au2) ¼ 2.683, r(Au2 –Au3) ¼ 2.856, r(Au2 –Au5) ¼ 2.856, r(Au2 –Au6) ¼ 2.683, r(Au5 –Au6) ¼ 2.683 A
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breaks down this assumption [66] and the definitive answer actually demands more accurate coupled cluster calculations meaning thus that the problem of which conformer of thiophenol is the most stable still remains an open one. However, the fact that the rotational barrier of thiophenol which separates both conformers from each other is of the order of RT should be kept in mind when investigating this problem both experimentally and theoretically. Such a small barrier implies that both conformers are present at room temperature and therefore precludes the experimental identification of a unique conformer at this temperature. On the other hand, without temperature-dependent experimental studies, it is not clear which of the computational levels, DFT or coupled clusters and their many variants, provides the most accurate description. Moreover, a small height of the rotational barrier may lead to some inaccuracy in evaluating the thermodynamic properties of thiophenol using the RRHO model. The later source of error will be present whatever is the accuracy of the computational level used to compute the electronic structure. Using the DF methods, we have thoroughly studied the geometrical, spectroscopic and energetic properties of both conformers of thiophenol, primarily focusing on the S –H group and its bond dissociation energy. To shed a light on the S– H bond cleavage in thiol-gold clusters, the lower-energy portions of the PESs of thiophenol with the gold clusters Au5 and Au6 have been investigated. We have demonstrated that the sulfur atom prefers to anchor to two- and three-coordinated atoms of gold in these clusters to form a strongly directional gold – sulfur bond. The hydrogen abstraction from the S– H group bonded to the two-coordinated gold atom in Au5 results in the formation of a bridging Au – S dibond which leads to a spectacular reduction of the bond dissociation energy of thiophenol by almost a factor of three.
Acknowledgements This work was supported by the Region Wallonne (RW. 115012) and used the computational facilities of NIC (University of Liege). The work of F. R. was also partially supported by ARC (University of Liege).
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Thiophenol and thiophenol radical and their complexes with gold clusters Au5 and Au6 F. Remaclea,*, E.S. Kryachkoa,b,1 a
Department of Chemistry, Bat. B6c, University of Liege Sart-Tilman, B-4000 Liege 1, Belgium b Bogoliubov Institute for Theoretical Physics, Kiev 03143 Ukraine Received 20 February 2004; revised 22 February 2004; accepted 22 February 2004 Dedicated to Prof. Galyna Puchkovskaya Available online 28 July 2004
Abstract The longstanding controversy between experiment and theory regarding which conformer of thiophenol, planar or perpendicular, is the most stable and what is the magnitude of the corresponding rotational barrier of the S– H group is discussed. We propose a variety of rather modest high-level computational methods within the density theory, which corroborate the experimental data. These methods demonstrate that the planar structure of thiophenol is the most stable and the magnitude of the rotational barrier falls within the experimental range of 3.35 ^ 0.84 kJ mol21. However, the barrier is of the order of RT at room temperature, which might prevent to clearly identify the most stable conformer of thiophenol in experiments and leads to a large-amplitude motion of the thiolic hydrogen. On the other hand, such low value of the barrier may lead to some error in evaluating the thermodynamic properties of thiophenol within the rigid-rotor-harmonic oscillator model, in particular for the bond dissociation enthalpy. We also show the existence of a large entropy contribution to the Gibbs free energy difference between the planar and perpendicular conformers which is the order of the rotational barrier (<4 kJ mol21). This might be of interest for experimental study. The most stable complexes of thiophenol with the gold clusters Au5 and Au6 are also investigated. It is shown that the sulfur atom prefers to anchor to two- and three-coordinated atoms of gold in these clusters to form a strongly directional gold – sulfur bond. The hydrogen abstraction from the S– H group of thiophenol bonded to the two-coordinated gold atom in Au5 yields the bridging Au –S dibond and results in a spectacular reduction of the bond dissociation energy of thiophenol by nearly a factor of three. q 2004 Elsevier B.V. All rights reserved. Keywords: Thiophenol; S–H bond; Rotational barrier; Thiophenol radical; Bond dissociation energy; Gold–sulfur bond; S– H cleavage
1. Introduction: thiophenol Thiophenol (benzenethiol) ArSH as a derivative of phenol where the oxygen atom is substituted by the sulfur belongs to the sulfur-containing compounds whose importance in atmospheric, environmental, biological, and medical areas can hardly be overestimated (see Refs. [1 – 4] for recent works and references therein). Moreover, the recent huge activity in nanoscience and nanotechnology has renewed the interest in these compounds, particularly in thiophenols, because of the ability of sulfur to bind gold clusters (see Refs. [6 – 18] and references therein). The first studies of thiophenol go back to 1956 when Scott et al. [19] * Corresponding author. Tel.: þ 32-4-366-2341; fax: þ32-4-366-3413. E-mail address: [email protected] (F. Remacle). 1 Co. Corresponding author. [email protected] (E.S. Kryachko). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.02.056
(see also Ref. [20]) investigated its thermodynamic properties and obtained a value of 3.35 ^ 0.84 kJ mol21 for the barrier height Vrot of the internal rotation of the S –H group from the stable planar to the transition-state perpendicular conformer. In 1974, Larsen and Nicolaisen [21] (see also Refs. [22,23]) reported for the first time the experimental microwave and far-infrared spectra of thiophenol and estimated Vrot at 3.2 kJ mol21 (3.4 kJ mol21 in the NMR study in CCl4 solution [24 – 26]; see also Refs. [27 –29]). What matters and is relevant for the present work is that these studies have concluded that the thiophenol molecule exhibits a planar structure. Note, however, that the reported experimental barrier is small and becomes of the order of RT at room temperature. This could prevent to clearly identify the most stable conformer of thiophenol. Strictly speaking, the experimental structure of thiophenol has not been determined so far. According to
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the microwave experiments by Johansson et al. [22] carried out in 1967, thiophenol is a planar molecule. This planarity has been confirmed by the early lower-level quantum chemical calculations [23,30 –32] that estimated a rotational barrier between 13.9 and 16.7 kJ mol21. On the other hand, in 1979 Lunazzi et al. [27] suggested that thiophenol is nonplanar and characterized by a twist angle of 26 ^ 38. They argued that the low rotational barrier does not prevent large-amplitude motion of the thiolic hydrogen atom far from its equilibrium position, with the mean value of the twist angle reaching 30– 408 (see also Refs. [33,34]). The 20 years following 1974 were marked by a puzzling disagreement between experiment and high level ab initio theory [35 – 37]. For instance, a rather high MP2/ 6-311G(d,p)//HF/6-311G(d,p)(5D) computational method favored a perpendicular orientation of the S – H group relative to the phenyl ring over the planar by 1.95 kJ mol21 [37]. During this period the question of whether a minimum computational level there exists, say within the density functional theory (DFT), that accurately corroborates the experimental data on the planar conformer being the most stable one and on the magnitude of Vrot was regrettably left unanswered. Moreover, these issues have almost been forgotten in the last decade, except the work by Wiberg and Rablen [38] who demonstrated in 1998 that the MP2/ 6-311þ þ G(d,p) favors the perpendicular form ArSH’ and estimates the rotational barrier as equal to 2.1 kJ mol21. Nowadays, numerous computational works on a variety of thiophenol properties are conducted worldwide. They in particular focus on evaluation of its so called homolytic bond dissociation energy as one of key tool in determining antioxidant activity [39 – 41] (BDE; see below for the definition; see also Ref. [4] for current review) and on the formation of the gold – sulfur bond (Refs. [9,11 –14,16] and references therein) resulting in the S– H group cleavage. The latter still remains a controversial issue (see Refs. [5 – 8, 10,15,18,42] and references therein). It thus seems worth to finally resolve the aforementioned discrepancy between experiment and theory and to show which consequences can be then drawn. This is the goal undertaken in the present work.
keyword and the ‘Ultrafine’ integration. Single-point (sp) calculations were also carried at the B3LYP/ 6-311þ þ G(2df,2p) (; D)//B3LYP/A level. The harmonic vibrational frequencies and corresponding unscaled zero-point vibrational energies (ZPVE) were calculated at all employed DF/X levels (X ¼ A, B, and C) in order to distinguish the topology of the stationary points and to obtain the thermodynamic properties of the studied complexes. All reported relative energies include the ZPVE correction. Many computational studies on the thiol-gold clusters [46] suggested the basis set superposition error to be negligible (less than 0.015 eV) on the relevant accuracy scale related particularly with the usage of the RECPs and is therefore not taken into account in the present work.
3. Thiophenol: geometries, Vrot and BDE Some selected properties of thiophenol in its planar (Fig. 1) and perpendicular conformers necessary for a further discussion are gathered in Table 1. We first note that the B3LYP/A geometry of the planar conformer is almost equivalent to that optimized at the higher B3LYP/ 6-311þ þ G(d,p) level in Ref. [47] and that the C1 – S7 ˚ at the MP2/6-31 þ G(d) [38] bond length of 1.7753 A satisfactorily matches the present value. The agreement with the experimental microwave geometry [22]: rexpt(C1 –S7) ¼ ˚ and /exptC1S7H7 ¼ 998 is ˚ , rexpt(S7 – H7) ¼ 1.30 A 1.77 A also rather fair. Interestingly, the early measured rotational constants A ¼ 5588:0; B ¼ 1577:7; and C ¼ 1231:1 MHz [22] do not allow to distinguish between the two conformers because, as follows from Table 1, their computed values are very close: DA ¼ 9 – 11 MHz, DB ¼ 18 – 20 MHz, and DC ¼ 1 – 2 MHz.
2. Computational framework All calculations of thiophenol and thiophenol (thiyl) radical and their complexes with gold clusters, conducted in the present work, were performed with the density functionals (DFs) B3LYP, MPW1K [43], BLYP in conjunction with a variety basis sets 6-311G(d,p) (; A), 6-311 þ G(2d,2p) (; B), and cc-pVDZ (; C) (for ArSH and ArSz) and one of the mostly employed energy-consistent 19-electron relativistic effective core potential (RECP) developed by Ermler, Christiansen and co-workers [44] using GAUSSIAN 03 package of quantum chemical programs [45]. These optimizations were conducted with the ‘Tight’
Fig. 1. View of the thiophenol molecule and its relevant atomic numbering.
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167
Table 1 ˚ ), bond angles (in 8), rotational constants A; B; and C (in MHz), total dipole moment (in D), S –H stretching frequency (in cm21 followed The bond lengths (in A by the IR activity in km mol21 in parentheses), imaginary transition frequency nts (in ı cm21, only for the ArSH’ conformer), electronic energy E (in hartree), ZPVE (in kcal mol21), enthalpy H (in hartree) and entropy S (in cal mol21T21) of the thiophenol ArSH in the stable planar ArSH and transition-state perpendicular ArSH’ conformers and thiophenoxyl radical ArS· B3LYP/A
B3LYP/B
B3LYP/C
MPW1K/A
MPW1K/B
MPW1K/C
BLYP/A
BLYP/B
BLYP/C
ArSH r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C4 –C5) r(C5 –C6) r(C1 –C6) r(C1 –S7) r(S7 –H7) /C1 S7 H7 A B C D nCS nCC a nSH E þ 630 Esp þ 630 ZPVE þ 60 H þ 630 S þ 80
1.398 1.392 1.393 1.393 1.391 1.398 1.789 1.347 96.8 5613.2727 1564.6523 1223.5876 1.23 409 (0.5) 1112 (30) 2680 (2) 20.521611 20.547056 1.98 20.416396 1.56
1.395 1.390 1.391 1.391 1.389 1.396 1.784 1.340 97.0 5633.8077 1570.7138 1228.2702 1.08 409 (0.5) 1113 (31) 2674 (0.5) 20.538067
1.403 1.397 1.397 1.398 1.396 1.404 1.790 1.357 96.8 5569.8338 1556.0929 1216.2879 1.05 410 (0.5) 1115 (30) 2668 (2) 20.470375
1.402 1.396 1.397 1.397 1.395 1.403 1.783 1.350 96.5 5579.8115 1563.9461 1221.5594 1.00 401 (0.5) 1086 (33) 2610 (0.4) 20.504904
1.409 1.402 1.403 1.404 1.401 1.411 1.790 1.365 96.8 5520.2724 1549.6171 1209.9635 1.00 401 (0.5) 1089 (32) 2613 (2) 20.440132
1.414 1.407 1.408 1.408 1.406 1.415 1.807 1.369 96.8 5488.0586 1531.9578 1197.6436 0.95 395 (0.8) 1073 (26) 2568 (4) 20.336954
2.11 20.365058 0.13
0.65 20.401831 0.04
0.67 20.337012 0.14
1.408 1.402 1.403 1.403 1.401 1.409 1.807 1.358 96.5 5531.5735 1539.5370 1204.3458 1.16 393 (0.9) 1066 (31) 2583 (4) 20.396557 20.421127 0.11 20.294203 1.35
1.406 1.400 1.401 1.402 1.399 1.407 1.800 1.351 97.0 5550.7087 1545.9646 1209.1862 1.01 394 (0.8) 1068 (28) 2578 (1) 20.412466
2.20 20.432609 0.10
1.404 1.398 1.398 1.399 1.396 1.405 1.788 1.356 96.3 5562.1472 1557.7796 1216.9506 1.15 401 (0.6) 1086 (31) 2620 (2) 20.488924 20.512918 0.54 20.385951 0.66
0.34 20.309837 0.36
0.24 20.234484 0.36
ArSH’ r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C1 –S7) r(S7 –H7) /C1S7H7 /C4C2S7C6 A B C D nCS nCC a nSH nts E þ 630 Esp þ 630 ZPVE þ 60 H þ 630 S þ 76 DEðArSH’ – ArSHÞ DHðArSH’ – ArSHÞ DG298 K ðArSH’ – ArSHÞ
1.397 1.393 1.393 1.809 1.351 97.1 1.0 5602.7115 1546.3636 1222.1014 1.8 408 (0.2) 1106 (4) 2638 (7) 20 20.521297 20.545462 1.93 20.417003 0.61 0.83 21.6 4.6
1.394 1.391 1.391 1.804 1.344 97.1 1.0 5623.6884 1551.1467 1225.9722 1.7 408 (0.3) 1107 (3) 2638 (2) 118 20.536800
1.402 1.398 1.398 1.811 1.360 96.8 1.1 5558.7332 1537.2430 1214.4645 1.6 407 (0.2) 1107 (3) 2629 (2) 112 20.469064
1.400 1.397 1.397 1.806 1.353 96.6 1.1 5568.4547 1542.9358 1218.3807 1.7 398 (0.2) 1078 (3) 2578 (2) 148 20.502706
1.407 1.404 1.404 1.812 1.368 96.4 1.1 5510.5249 1529.7390 1207.6133 1.6 398 (0.1) 1079 (3) 2575 (2) 140 20.437995
1.412 1.408 1.408 1.829 1.372 97.1 1.0 5479.4434 1512.0027 1195.1229 1.5 391 (0.1) 1063 (4) 2528 (12) 139 20.335016
1.97 20.364714 0.64 3.44 (2.85) 0.9 5.3
0.47 20.400596 1.13 5.77 (5.04) 3.2 6.9
0.48 20.335865 1.18 5.61 (4.81) 3.0 6.7
1.406 1.403 1.403 1.828 1.362 96.9 1.2 5522.6358 1520.5102 1202.3200 1.8 389 (0.1) 1062 (5) 2541 (11) 88 20.395719 20.419063 0.03 20.294292 1.25 2.20 (1.87) 20.2 4.9
1.405 1.401 1.401 1.822 1.355 97.1 1.0 5542.5770 1525.7589 1206.4287 1.7 391 (0.2) 1063 (4) 2542 (4) 140 20.410664
2.09 20.432269 0.56 3.33 (2.86) 0.9 5.3
1.402 1.399 1.399 1.810 1.359 96.5 1.2 5551.6844 1538.1954 1214.7166 1.8 397 (0.2) 1077 (4) 2583 (7) 97 20.487725 20.510431 0.41 20.385694 1.17 3.14 (2.63) 0.7 5.0
0.20 20.308984 1.18 4.73 (4.11) 2.2 6.2
0.07 20.233526 1.23 5.09 (4.36) 2.5 6.4
ArSz r(C1 –C2) r(C2 –C3) r(C3 –C4) r(C1 –S7) E þ 629 Esp þ 629 ZPVE þ 54 H þ 629
1.401 1.392 1.393 1.771 20.876612 20.901068 2.56 20.780869
1.398 1.389 1.391 1.767 20.891987
1.422 1.391 1.403 1.733 20.842738
1.422 1.388 1.402 1.725 20.874001
1.429 1.395 1.409 1.732 20.812801
1.434 1.400 1.414 1.744 20.717154
2.72 20.746782
1.33 20.780099
1.37 20.718849
1.430 1.394 1.409 1.740 20.775496 20.797242 0.95 20.682185
1.427 1.392 1.407 1.737 20.788847
2.77 20.795944
1.425 1.390 1.405 1.728 20.860544 20.881794 1.24 20.766774
1.14 1.04 20.695266 20.623719 (continued on next page)
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Table 1 (continued)
S þ 77 DEðArSH – ArSz Þb DEðArSH – ArSz Þb,c DHðArSH – ArSz Þb
B3LYP/A
B3LYP/B
B3LYP/C
MPW1K/A
MPW1K/B
MPW1K/C
BLYP/A
BLYP/B
BLYP/C
0.96 358.0 360.6 333.2
0.89 360.8
0.87 312.6
1.45 321.5
1.45 312.1
1.46 292.8
288.0
297.4
288.1
1.53 296.3 303.7 272.5
1.43 302.8
336.1
1.51 314.9 322.1 290.7
279.0
269.0
21
The energy, enthalpy and Gibbs differencies are given in kJ mol . The numbering of atoms is indicated in Fig. 1. The superscript sp corresponds to the DF/D//DF/A computational level after the DF/A unscaled ZPVE. a nCC was incorrectly assigned to nCS ¼ 1120 cm21 in Ref. [59]. In fact, this is the combinational mode of the phenyl ring composed of nCC ; dCH ; and dCCC (see Refs. [4,65] and references therein). b The exact energy of 20.5 hartree of the ground-state hydrogen atom is used to avoid the well-known self-energy problem of the employed DFT potentials (see also Refs. [53,58]). c The BDE is evaluated at the DF/D//DF/A computational level taking the DF/A unscaled ZPVE into account.
The main difference between the geometries of the two conformers of thiophenol is the length of the C1 –S7 bond that elongates under the planar ) perpendicular transition ˚ . The S 7 – H 7 bond stretches by only by < 0.02A ˚ that redshifts its stretching vibrational 0.001 – 0.004 A mode nSH by about 40 cm21 (Table 1). Note that the nSH stretch was experimentally observed in the liquid phase at 2584 cm21 [19]. It is worth mentioning a small nonplanarity of the perpendicular form manifested in that the C1 – S7 bond lies out of the phenyl ring by ca. 18 (Table 1), although it might be attributed to a deficiency of all employed computational levels. Some discrepancies in the geometries of both forms are related to the employed computational level (without any reference to the undetermined experimental geometry): (i) BLYP systematically overestimates the bond lengths compared to B3LYP and MPW1K. For C –C bonds, this discrepancy falls within a ˚ . This margin doubles for the narrow margin of 0.01 A C1 – S7 bond. (ii) Within a given density functional, there arises a sort of basis discrepancy. It means that for both aforementioned bonds, the basis A systematically ˚ compared overestimates their lengths by 0.001– 0.005 A to B and underestimates them within nearly the same margin in comparison to C. These issues could only be resolved by a straightforward comparison with an experimentally determined geometry. Interestingly, all employed computational methods favor the planar conformer over the perpendicular although their energetic offset Vrot varies from method to method (Table 1). If, for instance, B3LYP/A estimates it as 0.8 kJ mol21 (but B3LYP/D//B3LYP/A gives 4.2 kJ mol21), B3LYP/B and B3LYP/C fairly agree with the experimental magnitude expt ¼ 3:35 ^ 0:84 kJ mol21 of the rotational barrier Vrot [19,20]. The MPW1K in conjunction with the basis A falls into the same range of Vrot whereas its usage with the bases B and C (including the single-point MPW1K/D// MPW1K/A) overestimates the barrier height. The BLYP with the bases B and C (also together with BLYP/ D//BLYP/A) shows a similar tendency although with A, on the contrary, it underestimates Vrot : The potential energy function of the internal rotation of the S7 – H7 group by angle u around the C1 –S7 bond
can be rather accurately approximated by VðuÞ ¼ Vrot sin2 u [21,33,36]. Assuming that this internal rotation of the S7 – H7 group is well separated from the other vibrational modes of thiophenol, one can write the corresponding Schro¨dinger equation "2 00 c þ ðEn 2 Vrot sin2 uÞcn ¼ 0 2I n
ð1Þ
for the nth-state wavefunction cn ðuÞ characterized by the eigenenergy En : In Eq. (1), I is the corresponding moment of inertia, cn 00 denotes the second derivative of cn with respect to u: Eq. (1) is the Mathieu equation [48],
cn 00 þ ½1n þ 2q cosð2uÞcn ¼ 0
ð2Þ
where
2I 1 V E 2 n 2 rot "2 I 2q ¼ 2 Vrot : " 1n ¼
According to Table 2 of Ref. [21], 2q ¼ 13:688 so one can use the approximate formula [49] for the eigenvalues 1n : pffiffi 1n < 22q þ 2ð2n þ 1Þ q ð3Þ that in turn determines the equidistant spectrum 1 Vrot En ¼ n þ pffiffi : 2 q
ð4Þ
expt Substituting Vrot ¼ 3:35 ^ 0:84 kJ mol21 ¼ 280 ^ 70 cm21 and 2q ¼ 13:688 into Eq. (4), one obtains that E0 ¼ 53:5 ^ 13:5 cm21 and E1 ¼ 160:5 ^ 40:5 cm21. The difference E1 2 E0 determines the torsional frequency nt ¼ 107 ^ 27 cm21 of the S7 –H7 group. The obtained value of nt is in a fairly good agreement with the range of 93 –128 cm21 calculated at all employed computational levels, except the B3LYP/A and BLYP/A, which underestimate nt giving 46 and 74 cm21, respectively, and 21 the observed nexpt (see Fig. 1 and Table 3 of t ¼ 90:9 cm Ref. [21]). The computed entropies of the planar and perpendicular forms of thiophenol are given in Table 1. Their difference is quite large, between 3 and 5 cal mol21 T21 which gives rise
F. Remacle, E.S. Kryachko / Journal of Molecular Structure 708 (2004) 165–173
to a large entropy contribution (about 4 kJ mol21) to the Gibbs free energy at room temperature (Table 1). The latter expt is comparable with Vrot and hence such large entropy effect might be observable experimentally. The fact that the experimental magnitude of the rotational barrier is of the order of RT at T ¼ 298 K, viz., 3.2 vs. 2.5 kJ mol21, may lead to some error in evaluating the thermodynamic quantities within the rigid-rotor-harmonic-oscillator (RRHO) model [50,51], which is implemented in the GAUSSIAN 03 suit of programs [45]. This may further result in an insufficient accuracy for estimating the so-called bond dissociation enthalpy of thiophenol and comparing it with the experimental one. Let us nevertheless neglect this error since it deserves a separate study and analyze the values of the bond dissociation energy of the S – H group of thiophenol that we compute. The simplest definition of the bond dissociation energy of the S – H bond of thiophenol (BDEthiophenol(S – H) for shorthand; for the complete definition of the bond dissociation energy and enthalpy see Refs. [4,39 – 41]) is the ZPVE-corrected energy difference between the ground state of ArSH and that of the radical ArSz. Some relevant properties of the latter are gathered in Table 1. One observes that the abstraction of H7 strengthens the C1 –S7 bond due to a stronger conjugation effect that increases the double-bond character. This in turn slightly weakens the neighbouring C1 – C2 and C1 –C6 bonds. The difference in the energies and enthalpies of ArSH and ArSz (last two lines of Table 1), both computed with the unscaled ZPVE, show a strong dependence on the computational level. The differences between the different levels fall within a range of < 70 kJ mol21. The upper-bound estimate to the BDEthiophenol(S– H), equal to 360.8 kJ mol21, is obtained within the B3LYP/B method whereas the lower-bound of 292.8 kJ mol21 is given by BLYP/C. The MPW1K DF potential provides a value of BDEthiophenol(S –H) within a range of 312.1 – 321.5 kJ mol21 while BDE amounts to 322.1 kJ mol21 at MPW1K/D// MPW1K/A after the MPW1K/A ZPVE. The corresponding enthalpy difference ranges from 272.5 kJ mol21 (BLYP/A) to 336.1 kJ mol21 (B3LYP/B). It completely falls within the wide range of about 16 kJ mol21 (precisely, after some correction, from 331.0 to 349.4 kJ mol21 [52]) obtained for the available experimental values [53 –57] of this quantity and its early reported theoretical estimates [54,58,59] (see also Table 1 in Ref. [58]).
4. Complexes of thiophenol and thiophenyl radical with gold clusters Au5 and Au6 In this Section, we discuss ArSH –Au5#n # 6 complexes as prototypical model systems to study the gold –sulfur bond between arylthiol molecules and gold clusters. The latter are chosen in their planar electronic ground states [60 – 62]. The potential energy surface (PES) of ArSH – Au5 presents three stable conformers and their rotamers. Fig. 2
169
shows that the conformer I 5 and its rotamer II 5 , distinguished from I5 by the orientation of phenyl ring relative to the plane of the gold cluster, and other two conformers III5 and IV5. There also exist one rotamer of III5 and three rotamers of IV5 (not shown in Fig. 2). I5 is slightly more stable by 0.7 kJ mol21 than II5 (Table 2). I5 and III5 have practically the same electronic energy. IV5 has an energy 28.3 kJ mol21 higher than I5. All mentioned structures are highly polar (7.4, 7.0, 8.1 and 5.6 D). In I5 and II5, the sulfur atom is anchored to threecoordinated atom Au1 (Fig. 2) and forms the covalent ˚ long. Compared to the bare cluster, Au1 – S7 bond of 2.47 A the gold cluster Au5 in I5 and II5 undergoes quite drastic geometrical changes, in contrast to the thiophenol molecule where major ones occur at its C1 – S7 and S – H bonds. The ˚ while the latter by former is elongated by 0.024 –0.025 A ˚ only 0.004 A. This is accompanied by blue shift (4 – 5 cm21) of the nCS stretch and by a red shift of 42 –46 cm21 predicted for nSH : Note that the former is found at 404 – 405 cm21 which correlates with 420 cm21 observed in a monolayer of thiophenol on silver [63] (see also Ref. [64]). The most remarkable change taking place in thiophenol under the formation of the gold –sulfur bond affects the S – H bending: the S– H bond is rotated by the dihedral angle /H7S7C3C5 ¼ 72.08 in I5 and 72.18 in II5, respectively. The Au –S bond favors the quasi-perpendicular conformer of thiophenol and shows a strong directionality: the angle /C1S7Au1 varies within 110.8 –111.18. A similar interval of the Au – S –C bond angle in the CH3S– Aun has recently been found in Ref. [45]. The gold –sulfur bond in I5 and II5 is characterized by the stretching vibrational mode nAuS centered at 316– 317 cm21. The binding energies of I5 and III5 amount to 54.2 kJ mol21 despite the fact that in the latter, the Au – S bond is slightly longer although herein the S –H group is anchored to the two-coordinated atom of gold. Let us now consider the lower-energy portion of the PES of ArSH – Au6 comprising of two conformers I6 and II6 displayed in Fig. 3. As expected, since the sulfur atom is anchored to the two-coordinated gold atom in I5 and I6, they are characterized by similar geometrical patterns in the vicinity of the gold – sulfur bond (compare Figs. 2 and 3). Furthermore, the latter has a rather similar binding energy, viz., 51.5 kJ mol21. A covalent Au – S bonding of sulfur to the four-coordinated gold atom yields the complex II6 which, by analogy with IV5, is less stable by 32.8 kJ mol21 compared to I6. The radical structures I5 and Iz6 resulting from the cleavage of the S – H bond in their parent complexes I5 and I6 are displayed in Figs. 2 and 3 together with their geometrical parameters (see also Table 2). The most interesting is the Iz5 radical where the sulfur atom is anchored to the three-coordinated Au bridge. The formation of such bridging Au –S dibond between the thiophenol radical ArSz and the gold cluster Au5 significantly (by factor
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˚ and bond angles in 8. Fig. 2. Two stable complexes of thiophenol and thiophenyl radical with cluster of five gold atoms. The bond lengths are given in A The optimized bond lengths in the W-shape cluster Au5 are the following: rðAu1 – Au2 Þ ¼ 2:740; r(Au1 – Au4) ¼ 2.831, r(Au1 – Au5) ¼ 2.722, ˚ [62]. r(Au3 –Au4) ¼ 2.678 A
of three) reduces the BDE(S – H), which is equal to 113.2 kJ mol21. This is interesting effect, reported in the present work for the first time, is due to the unexpectedly strong influence of the bridging Au – S dibond on BDEthiophenol(S– H). It might help to resolve
the longstanding controversy on to the adsorption of the thiol S – H group on gold. One the one hand, IR and Raman spectroscopy [6,7,15] have shown that the S –H stretching mode of thiols adsorbed on gold is absent. This is believed to be due to the dissociation of the S – H bond and
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171
Table 2 The electronic energy E (in hartree), ZPVE (in kcal mol21), enthalpy H (in hartree), entropy S (in cal mol21 T21) and relevant spectroscopic features (for notation see the legend to Table 1) of the ArSH–Au5#n # 6 and ArSz –Au5#n # 6 calculated in the present work
E ZPVE H S nSAu nCS nSH
I5
II5
III5
IV5
Iz5
I6
II6
Iz6
21310.467188 64.23 21310.347088 167.78 316 404 2638 (5)
21310.466900 64.20 21310.346836 167.41 317 405 2634 (3)
21310.467180 64.16 21310.347132 171.26 313 405 2632 (4)
21310.457100 64.07 21310.337071 170.55 121 404 2663 (5)
21309.915542 58.86 21309.804934 156.10 281, 352 416
21446.499154 64.51 21446.376247 182.17 312 405 2633 (4)
21446.486268 64.27 21446.363542 180.80 108 405 2670 (4)
21445.882438 58.78 21445.769127 180.40 350 416
an adsorption of hydrogen [53]. On the other hand, vibrational and X-ray photoelectron spectroscopy studies have demonstrated that, e.g. CH3SH on Au(111) does not dissociate via a S –H bond cleavage [5]. We suggest here that the S – H bond cleavage strongly depends on the adsorption site where thiophenol (or generally speaking, thiol) is attached to gold. If it mimics the I5-type complex, which significantly reduces the BDEthiophenol(S –H) so that it becomes comparable with the formation energy of the bridging Au – S dibond, the S –H bond is likely to dissociate and H to adsorb on gold. Clearly, this process is also determined by the shape of the transition barrier. In other cases, like for example for Iz6 where the BDE(S– H) ¼ 306 kJ mol21 is still large, the S –H bond is not broken unless the thiophenol migrates to another site.
5. Summary In the present work we have demonstrated that the longstanding controversy between experiment and theory on the relative stability of the planar or perpendicular conformers of thiophenol can be resolved using the minimum basis sets A –D within the density functional approach. All employed density functional computational methods predict that the most stable conformer is planar and the computed value of the rotational barrier Vrot is in agreement with experimental data. It is usually assumed that the DF methods, particularly the B3LYP and MPW1K employed in the present work in conjunction with rather large basis sets such as B, C and D, are more reliable than the MP2 method. However, some conformational problems
˚ and bond angles in 8. The optimized bond lengths Fig. 3. Two stable complexes of thiophenol and thiophenyl radical with Au6. The bond lengths are given in A ˚ [62]. in the cluster Au6 are the following: r(Au1 –Au2) ¼ 2.683, r(Au2 –Au3) ¼ 2.856, r(Au2 –Au5) ¼ 2.856, r(Au2 –Au6) ¼ 2.683, r(Au5 –Au6) ¼ 2.683 A
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breaks down this assumption [66] and the definitive answer actually demands more accurate coupled cluster calculations meaning thus that the problem of which conformer of thiophenol is the most stable still remains an open one. However, the fact that the rotational barrier of thiophenol which separates both conformers from each other is of the order of RT should be kept in mind when investigating this problem both experimentally and theoretically. Such a small barrier implies that both conformers are present at room temperature and therefore precludes the experimental identification of a unique conformer at this temperature. On the other hand, without temperature-dependent experimental studies, it is not clear which of the computational levels, DFT or coupled clusters and their many variants, provides the most accurate description. Moreover, a small height of the rotational barrier may lead to some inaccuracy in evaluating the thermodynamic properties of thiophenol using the RRHO model. The later source of error will be present whatever is the accuracy of the computational level used to compute the electronic structure. Using the DF methods, we have thoroughly studied the geometrical, spectroscopic and energetic properties of both conformers of thiophenol, primarily focusing on the S –H group and its bond dissociation energy. To shed a light on the S– H bond cleavage in thiol-gold clusters, the lower-energy portions of the PESs of thiophenol with the gold clusters Au5 and Au6 have been investigated. We have demonstrated that the sulfur atom prefers to anchor to two- and three-coordinated atoms of gold in these clusters to form a strongly directional gold – sulfur bond. The hydrogen abstraction from the S– H group bonded to the two-coordinated gold atom in Au5 results in the formation of a bridging Au – S dibond which leads to a spectacular reduction of the bond dissociation energy of thiophenol by almost a factor of three.
Acknowledgements This work was supported by the Region Wallonne (RW. 115012) and used the computational facilities of NIC (University of Liege). The work of F. R. was also partially supported by ARC (University of Liege).
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