Influence of Hybridization and Cooperativity on the Properties of Au

ARTICLE pubs.acs.org/JPCA

Influence of Hybridization and Cooperativity on the Properties of Au-Bonding Interaction: Comparison with Hydrogen Bonds Qingzhong Li,* Hui Li, Ran Li, Bo Jing, Zhenbo Liu, Wenzuo Li, Feng Luan, Jianbo Cheng, Baoan Gong, and Jiazhong Sun The Laboratory of Theoretical and Computational Chemistry, Science and Engineering College of Chemistry and Biology, Yantai University, Yantai 264005, People's Republic of China

bS Supporting Information ABSTRACT: Quantum chemical calculations have been performed to study the hybridization effect in H2O
AuCH2CH3, H2O
AuCHCH2, and H2O
AuCCH dimers, and the cooperativity between the hydrogen bond and Au bonding in three trimers (T1, T2, and T3) composed of one AuCCH and two H2O molecules. With regard to the organic Au compounds, sp-hybridized AuCCH forms the strongest Au bonding, followed by sp2 and then sp3. The C
Au bond is elongated, and its elongation becomes larger with the increase of the s character in hybrid orbitals, whereas the corresponding stretch vibration displays a small blue shift. The positive cooperativity is present for the hydrogen bond and Au bonding in T1 and T2 trimers, whereas the negative cooperativity is found in T3 trimer. The results show that the hybridization effect and cooperative interaction in Au bonding are similar to those in hydrogen bonds. Additionally, an OH 3 3 3 Au hydrogen bond is suggested in T1 trimer.

1. INTRODUCTION Gold has become one of the most attractive coinage metals since it was reported that gold has a high activity for CO oxidation at low temperature.1 Now gold has been applied extensively in anticancer treatment,2 homogeneous and heterogeneous catalysis,3 and surface absorption.4 Hydrogen bonds play a crucial role in chemistry, physics, and biology.5 In view of the analogy in bonding between gold and hydrogen,6,7 it has been demonstrated that like hydrogen, Au can form Au bonding with lone-pair electron donors.8,9 The strength of hydrogen bond is not only related to the proton donor and acceptor atoms but also associated with other factors such as hybridization of atoms adjoined with them.10
15 Grabowski and co-workers14,15 studied the hybridization effect on the C
H hydrogen bond in the complexes of methane
Y, ethane
Y, and acetylene
Y (Y = water, ammonia, hydrogen sulfide) with theoretical methods and concluded that with regard to the hydrocarbons, sp-hybridized acetylene forms the strongest bond, followed by sp2 and then sp3. This is consistent with the acidity of C
H proton in the hydrocarbons. The cooperativity is an important property of hydrogen bonds, and the function of hydrogen bonds depends to a great extent on the cooperativity.16
21 The cooperativity of conventional hydrogen bonds has drawn considerable attention since the concept of a hydrogen bond was proposed. For example, the influence of polar near-neighbor on incipient proton transfer was investigated in H3N 3 3 3 HF and H3N 3 3 3 HF 3 3 3 HF complexes using experimental and theoretical methods.22 Then some studies also focused on the cooperativity of unconventional r 2011 American Chemical Society

hydrogen bonds. The cooperativity of CH 3 3 3 O blue-shifting hydrogen bonds in a cyclic trimer of formaldehyde and the effect of a methyl group on it have been investigated.23 Now the cooperativity between different types of interactions is fashionable.24
30 It has been concluded that the cooperativity is also present among different types of interactions and it is more prominent than that between the same type of interactions. Considering the similarity between hydrogen bonding and Au bonding, we want to study the influence of hybridization of the carbon atom in C
Au bond on Au bonding and the cooperativity between Au bonding and the hydrogen bond. We thus investigate the H2O
AuCH2CH3, H2O
AuCHCH2, H2O
AuCCH dimers and the trimers composed of one AuCCH and two water molecules with quantum chemcial calculations. Natural bond orbital (NBO) analysis has also been performed for these complexes.

2. COMPUTATIONAL DETAILS Geometrical structures of the dimers, trimers, and respective monomers were optimized using the MP2 method. The aug-ccpVTZ basis set is adopted for C, H, and O atoms, while the augcc-pVDZ-PP31,32 basis set is used for Au atom. Harmonic frequency calculation was then performed to affirm that these structures are local minima on the energy surfaces. The interaction energy has been calculated as the difference between the Received: November 11, 2010 Revised: February 18, 2011 Published: March 15, 2011 2853

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energy of the complex and the sum of energy of the monomers. It was corrected for the basis set superposition error (BSSE) using the Boys
Bernardi counterpoise scheme.33 A single-point energy calculation has been performed at the CCSD level on the MP2 geometry. The natural bond orbital (NBO) analysis was carried out for these complexes using the NBO package34 included in the GAUSSIAN 09 suite of programs. All calculations have been performed with the GAUSSIAN 09 program.35 We also performed an analysis of atoms in molecules (AIM) for T1 trimer using the AIM 2000 program.36

3. RESULTS AND DISCUSSION 3.1. Hybridization. Figure 1 shows the structures of H2O
AuCH2CH3, H2O
AuCHCH2, and H2O
AuCCH complexes optimized at the MP2 level. The structural, spectroscopic, and energetic parameters are given in Table 1. The C, Au, and O atoms are in a line in these complexes. The interaction energy is calculated with MP2 and CCSD methods. As expected, the MP2 method overestimates the interaction energy of Aubonding with respect to CCSD. The difference of the interaction energy between the two methods correlates with the hybridization of C atom. It increases in the order: C(sp3) < C(sp2) < C(sp). The interaction energy is calculated to be
56.04,
70.25, and
101.55 kJ/mol in H2O
AuCH2CH3, H2O
AuCHCH2, and H2O
AuCCH complexes, respectively, at the CCSD level. It is evident that the strength of Au-bonding depends on the hybridization of the C atom. The Au bonding becomes stronger in the order: C(sp3) < C(sp2) < C(sp). This is similar to that in C
H hydrogen bonds.10
15 The result indicates the similarity in the hybridization effect for Au bonding and hydrogen bonding. The positive charge on the Au atom is increased from AuCH2CH3 (0.332 e) to AuCCH (0.486 e). This indicates an increase of the acidity of the Au atom, resulting in an increase of its ability to accept electrons from water. Furthermore, the hybridization effect on Au-bonding is more prominent than that on hydrogen bonds. The MP2 interaction energy of Au-bonding almost doubles when the hybridization is changed from sp3 to sp. The methyl group plays a non-negligible role in hydrogen bonds.37 For CH 3 3 3 O hydrogen bond in the HCCH
H2O complex, the methyl group in the electron donor strengthens the hydrogen bond, whereas that in the proton donor weakens it.38 In methanol
AuCH3 complex, the methyl group in the electron donor also strengthens Au bonding.9 At the MP2 level, the interaction energy is
71.90 kJ/mol in the H2O
AuCH3 complex,9 while it is
60.44 kJ/mol in the H2O
AuCH2CH3 complex. This indicates that the methyl group in the electron acceptor also weakens the Au bonding. Thus the Au bonding has a similarity with hydrogen bonding in the regulating role of methyl group. The methyl group in the electron acceptor makes the interaction energy decrease by 2.15 and 11.46 kJ/mol for the CH 3 3 3 O hydrogen bond38 and Au bonding, respectively. The binding distance R(O 3 3 3 Au) is in a range of about 2.1
2.2 Å. This distance is smaller than the sum of the van der Waals Radii of Au and O atoms (about 3.0 Å
). The binding distance is 2.194 Å in H2O
AuCH2CH3 complex, 2.163 Å in H2O
AuCHCH2 complex, and 2.108 Å in H2O
AuCCH complex. Like in hydrogen bonds,15 a good relationship is present between the binding distance and the interaction energy. The shorter binding distance corresponds to the larger interaction energy.

Figure 1. Optimized structures of H3CCH2Au
H2O, H2CCHAuH2O, and HCCAu
H2O complexes at the MP2 level.

Table 1. Binding Distance (R, Å), Change of the C
Au Bond Length (Δr, Å), Shift of the C
Au Stretch Frequency (Δv, cm
1), Intensity of the C
Au Stretch Vibration (I, km/ mol), Interaction Energy Corrected with BSSE (ΔE, kJ/mol), and Wiberg Bond Index (WIB) of the O 3 3 3 Au Bond in H2O
AuCH2CH3, H2O
AuCHCH2, and H2O
AuCCH Complexes at the MP2 Level

R(O 3 3 3 Au)

H2O
AuCH2CH3

H2O
AuCHCH2

H2O
AuCCH

2.194

2.163

ΔrC
Au

0.004

0.015

2.108 0.021

ΔvC
Au

19

9

0

IC
Aua

8 (3)

3 (5)

18 (6)

ΔEb


60.44 (
56.04)


77.64 (
70.25)


117.76 (
101.55)

qAuc

0.261

0.357

0.593

μd WIB

1.00 0.132

1.82 0.148

3.43 0.181

a

Data in parentheses are the intensity of the C
Au stretch vibration in the respective monomer. b Data in parentheses are the interaction energy obtained at the CCSD level with a single-energy calculation on the MP2 geometry. c The charge on the Au Atom (q, e) in the isolated Au compound. d The dipole moment (μ, D) of the isolated Au compound.

Upon complex formation, the C
Au bond is elongated. The C
Au bond elongation is smallest in H2O
AuCH2CH3 complex and largest in H2O
AuCCH complex. The elongation of the C
Au bond has a correlation with the positive charge on the Au atom in the isolated molecules. With the increase of the positive charge on the Au atom, the acidity of the Au atom also increases, resulting in the increase of the elongation of the C
Au bond upon complexation. This is consistent with the interaction strength. Accompanied with the C
Au bond elongation, the C
Au stretch vibration displays a small blue shift in H2O
AuCH2CH3 and H2O
AuCHCH2 complexes. The C
Au stretch frequency almost has no change in H2O
AuCCH complex. A similar inconsistency also happens in some lithiumbonded complexes.39
41 However, the C
Au bond blue shift is smaller than the C
Li one. Feng et al39 attributed the inconsistency between the bond length change and frequency shift in F3CLi
H2O and F3CLi
N2 complexes to the coupling between the C
Li vibration and the vibration of other bonds like the 2854

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Table 2. Binding Distance (R, Å), Change of Bond Length (Δr, Å), and Shift of Stretch Frequency (Δv, cm
1) in the Complexes at the MP2 Levela D1 R(O 3 3 3 Au)

D2

D3

2.108

R(O 3 3 3 HC)

T1

T2

T3

2.092

2.103

2.111

(
0.016)

(
0.005)

(0.003)

2.184

2.323 (0.139)

R(O 3 3 3 HO)

1.722 (
0.224)

R(π 3 3 3 HO)

2.317

2.268 (
0.049)

Δr(C
Au)

0.021

0.002


0.002

0.023

0.021

Δr(C
H)

0.000

0.001

0.005

0.001

0.001

0.004

Δv(C
Au)

0


4

9


2

0

5

Δv(C
H)


8


10


58


9


16


50

0.020

a

Note: Data in parentheses are the difference in binding distance of the trimer relative to the dimer. Figure 2. Optimized structures of the studied trimers (T1, T2, and T3) and the related dimers (D1, D2, and D3) at the MP2 level.

C
F bond. Very recently, McDowell and Marcellin41 studied this anomalous correlation of bond extension with blue shift using a model derived from perturbation theory, and thought that the large dipole moment of F3CLi produces substantial bond extension, whereas the repulsion between the core electrons of the Li atom and the electrons of the bonding partner produces the blue shift of the stretching frequency. The dipole moment is not large for AuCH2CH3 (1.00 D) and AuCHCH2 (1.82 D). Although the dipole moment of AuCCH is 3.43 D, the C
Au stretch frequency suffers no change in the H2O
AuCCH complex. We think that the coupling between the C
Au vibration and the vibration of other bonds may be responsible for the blue shift. This can be validated in the displacement of all the atoms corresponding to the nominal C
Au stretching mode in the complexes and respective monomers (Table S2, Supporting Information). Hence, this shift is the result of mode mixing. We also analyzed the change in the intensity of this vibration in the complex (Table 1). One can see that the intensity of the C
Au stretch vibration is very weak. Upon complexation, it increases in H2O
AuCH2CH3 and H2O
AuCCH complexes but decreases in H2O
AuCHCH2 complex. However, the change in the intensity of the C
Au stretching in the monomer is consistent with the increase of the positive charge on the Au atom. The Wiberg bond index (WBI) for the Au 3 3 3 O bonding is also given in Table 1. The WBI value increases from 0.132 in H2O
AuCH2CH3 complex to 0.181 in H2O
AuCCH complex. This is consistent with the interaction energy. It is much smaller than that in covalent bonds, but it is much larger than that in hydrogen bonds.42 This supports the conclusion that the Au bonding exhibits a partially covalent nature.8,9 3.2. Trimers. The optimized structures of the trimers (T1, T2, and T3) and the related dimers (D1, D2, and D3) are illustrated in Figure 2. There is an OH 3 3 3 O hydrogen bond and a CAu 3 3 3 O bond in T1. Similar bonding and a π 3 3 3 HO hydrogen bond are present in T2. In T3, there is a CH 3 3 3 O hydrogen bond besides an Au bond analogous to that in T1. In all trimers,

AuCCH is represented with A, the H2O as the electron donor in Au bonding with B, and the other water with C, for convenience in the many-body energy analysis. The binding distance, change of bond length, and shift of stretch frequency are presented in Table 2. In T1, the Au 3 3 3 O binding distance is 2.092 Å. It shortens by 0.016 Å relative to that in D1. The binding distance of OH 3 3 3 O hydrogen bond is 1.722 Å in the trimer. It shortens by 0.224 Å relative to that in water dimer. Clearly, the shortening of the binding distance is more prominent for the hydrogen bond. In T2, the Au 3 3 3 O binding distance decreases by only 0.005 Å with respect to that in D1, while the π 3 3 3 HO binding separation contracts 0.049 Å relative to that in D2. In T3, the Au 3 3 3 O separation is 2.111 Å. It is larger by 0.003 Å than that in D1. The binding distance of the CH 3 3 3 O hydrogen bond is 2.323 Å in the trimer. This value is much larger than 2.184 Å in D3. The C
Au bond is elongated in D1, whereas the distant C
Au bond is shortened a little in D3 and elongated a little in D2. This bond suffers an elongation in all trimers. Its elongation is a little larger in T1 than in D1, whereas it is much larger in T2 than in D2. The C
H bond is lengthened in D3, whereas the distant C
H bond is almost not changed in D1 and is lengthened a little in D2. In all trimers, the C
H bond is also elongated. The C
H bond elongation in T3 is a little smaller than that in D3. The C
Au stretch vibration shows a small red shift in T1. A blue shift happens for the C
Au stretch vibration in T3. The blue shift in T3 is smaller than that in D3. In T2, however, the C
Au stretch almost has no change relative to the AuCCH monomer although a small red shift is found in D2. The C
H bond stretch vibration displays a red shift in all complexes. It is larger in T1 and T2 than in D1 and D2, respectively, while it is smaller in T3 than in D3. Table 3 presents the interaction energy in all trimers at the MP2 and CCSD levels. The CCSD value was obtained with a single-point energy calculation on the MP2 geometry. The geometries optimized for the dimers were used to calculate the interaction energy in the trimers. All interaction energies were corrected with BSSE. The MP2 values are more negative than the CCSD ones. The total interaction energy is calculated with the 2855

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Table 3. Total Interaction Energy (ΔEtotal, kJ/mol), Interaction Energy (ΔE, kJ/mol) of the Hydrogen Bond and Au Bonding, and Energy Change (ΔΔE, kJ/mol) Relative to the Dimers in the Trimers at the MP2 and CCSD Levelsa T1

T2

MP2

MP2

MP2

CCSD


165.78


143.92


137.13


118.17


124.56


107.42

ΔE(O 3 3 3 Au) ΔE(O 3 3 3 HO) ΔE(π 3 3 3 HO)


146.59 (
117.76)


125.60 (
101.55)


121.32 (
117.76)


103.80 (
101.55)


114.31 (
117.76)


99.12 (
101.55)


48.23 (
19.67)


42.60 (
18.69)
19.36 (
15.35)


16.62 (
13.92)


3.55


2.25


4.01


2.70

ΔΔE(O 3 3 3 HO) ΔΔE(π 3 3 3 HO)


28.82


24.05


28.55


23.91

ΔΔE(O 3 3 3 HC)


6.69 (
9.82)


5.77 (
7.92)

3.45

2.43

3.13

2.15

Note: Data in parentheses are the interaction energies in the dimers.

Table 4. Two-Body Interaction Energy (Etwo-body, kJ/mol), Three-Body Interaction Energy (Ethree-body, kJ/mol), Cooperative Energy (Ecoop, kJ/mol), and Total Interaction Energy (Etotal, kJ/mol) in the Trimers at the MP2 and CCSD Levelsa T1 MP2

CCSD

T2 MP2

CCSD

T3 MP2

CCSD

Etwo-body(A
B)
117.68
98.42
118.77
100.45
118.81
100.67 Etwo-body(B
C)
15.47
13.70
2.40
2.42
0.08
0.09 Etwo-body(A
C)
15.48
13.60
13.55
11.39
8.80

a

CCSD

ΔEtotal

ΔE(O 3 3 3 HC) ΔΔE(O 3 3 3 Au)

a

CCSD

T3


7.85

Ethree-body


20.67
19.84
3.71


3.41

2.10

1.93

Ecoop


28.35
23.68
4.01


2.70

3.02

2.05

Etotal


169.30
145.55
138.42
117.67
125.60
106.68

Note: Segments A, B, and C are seen in Figure 2.

formulas of ΔEtotal = EABC
(EA þ EB þ EC). It is
165.78 and
143.92 kJ/mol for T1 at the MP2 and CCSD levels, respectively. Both values are more negative than those in T2 and T3. This indicates that the former isomer is more stable than the latter two isomers. At the MP2 level, the interaction energy of the OH 3 3 3 O hydrogen bond is calculated to be
19.67 kJ/mol in water dimer. It is changed to be
48.23 kJ/mol in T1. This value in the trimer is increased by about 145% relative to the water dimer. The interaction energy of Au bonding in D1 is
117.76 kJ/mol. This value is increased to be
146.59 kJ/mol in T1. The increased percentage is about 24%. The CCSD method gives a similar increased percentage with the MP2 method for the Au bonding but a smaller increased percentage than the MP2 method for the OH 3 3 3 O hydrogen bond. The increase of the interaction energy for both types of interactions in T1 indicates that there is interplay between the two interactions in the trimer. Furthermore, the effect of Au bonding on the hydrogen bond is larger than that of the hydrogen bond on the Au bonding. These conclusions are similar to those in hydrogen bonds.43,44 A similar result also happens for T2. Compared with T1, the increased magnitude of the interaction energy in T2 is quite insignificant. At the MP2 level, the interaction energy of Au 3 3 3 O bonding in T2 is calculated to be
121.32 kJ/mol, only increased by 3.55 kJ/mol relative to D1. The interaction energy of π 3 3 3 HO hydrogen bonding in T2 is counted as
19.36 kJ/ mol, being merely larger 4.01 kJ/mol than that in D2.

For T3, the interaction energy between the two molecular pairs is less negative than that in the corresponding dimer. At the MP2 level, the interaction energy of Au bonding decreases by 3.45 kJ/mol, whereas that of the CH 3 3 3 O hydrogen bond decreases by 3.13 kJ/mol. The CCSD values are again smaller than the MP2 results. The result shows that there is negative cooperativity between the Au bonding and hydrogen bond in T3. 3.3. Many-Body Interaction Analysis. A many-body interaction analysis was also performed for all trimers. The two-body term (Etwo-body) is calculated to be a difference between the binding energy of each molecule pair in the trimer and the energy sum of the monomers with all of them frozen in the geometry of the trimer. The three-body term (Ethree-body) can be calculated as the total binding energy of the trimer minus the interaction energy of each pair of monomers with all of them frozen in the geometry of the trimer. The total binding energy of the trimer is the sum of two-body energy and three-body energy. The results are given in Table 4. All energies are corrected with BSSE. In T1, the EA-B is largest (
98.42 kJ/mol at the CCSD level), amounting to about 68% of the total interaction energy. The contribution from EB
C and EA-C is almost equal (9%) in the trimer. The Ethree-body is calculated to be
19.84 kJ/mol at the CCSD level. It amounts to about 12% and 14% of the total interaction energy at the MP2 and CCSD levels, respectively. This percentage equals to about 18% in the global minimum of the water trimer18
20 and 6% in H2O
HCN
H2O trimer.45 The former structure is cyclic, and the latter one is linear. The H2O
H2O
AuCCH trimer is also a cyclic structure, which can be validated in the following discussion. Hence the percentage contribution of the three-body term to the interaction energy is more prominent in the cyclic trimers.18
20 Furthermore, the Ethree-body is larger than the EB
C in the trimer. This is different from that in hydrogen bonds. The larger interaction energy between A and C molecules in T1 indicates that there is a direct interaction between the two molecules. We also provide some of the following evidence for it. The binding distance between the H atom in C and Au atom in A (about 2.8 Å) is smaller than the sum of the van der Waals radii for the H and Au atoms (about 3.3 Å). The topological analysis shows that a bond critical point (BCP) is present between the H atom in C and the Au atom in A. The electron density at the BCP is 0.010 au and the corresponding Laplacian is 0.035 au. Both values are in the range proposed for closed-shell interactions.46 This weak interaction between segments C and A makes the 2856

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Table 5. Atomic Charges (q, e) in the Monomers, Dimers, and Trimers q(O)B

q(H)B

H2O


0.9228

0.4614

AuCCH H2O
H2O


0.9581

0.4942

D1


0.9151

0.5077

q(H)A

q(C)Aa

q(Au)A

0.2172


0.8101

0.5930 0.4860

0.2123


0.7986

D2

0.2224


0.8421

D3

0.2426

T1


0.9523

0.5421

0.2123

T2


0.9176

0.5098

0.2208

T3


0.9147

0.5071

0.2324

0.6232 0.5778 0.4719


0.8161

0.4997 0.4810

a

The sum of the negative charges on the two C atoms in the AuCCH molecule.

structure more stable than possible linear structure. The O
H 3 3 3 Au hydrogen bond is proposed. Such interaction has been reported in a complex of formic acid with a three-gold cluster.47 The F
H 3 3 3 Au and N
H 3 3 3 Au hydrogen bonds were also observed in complexes of gold cluster with DNA bases48 and hydrogen fluoride.49 In these hydrogen bonds, gold clusters act as the proton acceptor. However, the covalent-bonded Au atom is the proton acceptor in T1. This suggests that organic Au atom has a similarity with a halogen atom.50 The result shows that the Au atom in the trimer can interact with the lone-pair electrons on the O atom of water and the proton of the other water simultaneously. We thus suggest that the covalent-bonded Au atom exhibits anisotropy like halogen atoms.51 In T2, the EA-B is
100.45 kJ/mol at the CCSD level, occupying about 85% of the total interaction energy. The percentage contribution of EA-C is about 10% and the minimal one, EB
C, takes up about only 2% of the total interaction energy due to the big distance. The Ethree-body is estimated to be
3.41 kJ/mol at the CCSD level, being much smaller than that in T1. The negative value of Ethree-body shows that there is positive cooperativity between the Au bonding and π hydrogen bond in T2. In T3, the contribution from the two-body interaction is still largest for A and B molecule pairs (94%), followed by A and C ones (8%), and that from B and C is very small. The value of Ethree-body is positive in the trimer, showing the negative cooperativity in the trimer. This result is different from that in T1 trimer, where the Ethree-body is negative and a positive cooperativity is present. A similar result is also found in the open forms of the water trimer.18
20 The negative percentage contribution of the three-body term to the interaction energy is 2% in T3. This contribution is smaller than that in the open forms of the water trimer.18
20 The cooperative energy (Ecoop) is the most effective method for evaluating the cooperativity of Au bonding and hydrogen bond. The Ecoop is calculated to be the difference between the total interaction energy in the trimer and the sum of interaction energy in the respective dimers. The result is also given in Table 4. The Ecoop value is negative in T1 and T2 but is positive in T3. The negative Ecoop indicates a positive cooperativity in T1 and T2, while the positive Ecoop shows a negative cooperativity in T3. The Ecoop absolute value at the MP2 level is larger than that at the CCSD level. The Ecoop is calculated to be
23.68 kJ/mol in T1 at the CCSD level. It amounts to about 16% of the total interaction energy in the trimer. The synergistic effect in T1 is more muted than that in the cyclic hydrogen-bonded trimer of HCHO,23 in which the synergistic effect is about 29%. The value

of Ecoop is small in T2 and T3 (
2.70 and 2.05 kJ/mol at the CCSD level, respectively). 3.4. NBO Analyses. People have different opinions on the origin of H-bonding synergistic effect. Kar and Scheiner thought that H-bonding cooperativity can mainly be attributed to the polarization induced in each subunit by the presence of its H-bonding partner.52 Glendening has pointed out that charge transfer could be regarded as the main source of cooperative stabilization and that polarization effect has only marginal influence on the cooperativity.53 In order to unveil the origin of the cooperativity between the hydrogen bond and Au bonding in all trimers, natural bond orbital (NBO) analyses have been carried out for all complexes. The electrostatic interaction plays a main role in the formation of hydrogen bond and Au bonding. The cooperativity can thus be understood with examining the charge change of corresponding atoms from the monomer to the dimer (Table 5). The positive charges on the H and Au atoms in AuCCH are 0.217 and 0.593 e, respectively. Both charges decrease in D1. The ability to accept electrons from the second water decreases for the H atom of AuCCH in D1. In D2, the sum of the negative charges on the two C atoms and the positive charge on the Au atom in AuCCH have an increase relative to the AuCCH molecule. This means that the electron-donating ability of the π electron and the ability to accept electrons for the Au atom also increase. Thus, there is a positive cooperativity in T2. In D3, the positive charge on the H atom of AuCCH increases but the positive charge on the Au atom decreases. The decrease of positive charge on the Au atom means that its ability to accept electrons decreases. Thus the negative cooperative effect is present for the hydrogen bond and Au bonding in T3. The decrease of the positive charge on the Au atom in D1 due to the formation of Au bonding is different from the increase of the positive charge on the H atom in D3 due to the formation of a hydrogen bond. The charge on the H atom in H2O is 0.461 e, whereas that on the O atom in H2O is
0.923 e. The former increases in D1, whereas the latter decreases in the dimer. Both charges become greater in water dimer. One can see that the change of the O atom in H2O is different in the hydrogen bond and Au bonding. The more positive charge on the H atom of H2O in D1 indicates an increase of its ability to accept electrons. The more negative charge on the O atom in water dimer shows an increase of its ability to donate electrons. Thus the positive cooperativity is present for the hydrogen bond and Au bonding in T1.

3. CONCLUSIONS Ab initio calculations have been performed to study the effect of hybridization and the cooperativity between Au bonding and the hydrogen bond on the properties of Au bonding. The results show that the strength of the Au-bonding interaction increases in the following sequence: C(sp3)
Au < C(sp2)
Au < C(sp)
Au. The C
Au bond is elongated, but the corresponding stretch vibration frequency has a blue shift. The influence of hybridization on the Au bonding is much more prominent than that on hydrogen bonds. In T1, there is an OH 3 3 3 Au H bonding besides an OH 3 3 3 O H bonding and an Au bonding. Its structure is cyclic and the cooperative energy is very large. A similar cooperative result is also found in T2 although the cooperative effect is much smaller. The Au and H atoms in AuCCH act as the electron acceptor in T3, simultaneously, thus the negative cooperativity is present for the hydrogen bond and Au bonding. It has been 2857

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The Journal of Physical Chemistry A demonstrated that the cooperativity of Au bonding exhibits similar properties with that in hydrogen bonds. In the future we will study the interaction of organic Au atoms with acidic and basic H atoms like in dihydrogen bonds based on the fact that the organic Au atom can provide the acidic and basic sites.

’ ASSOCIATED CONTENT

bS

Supporting Information. The C
Au stretch frequency and intensity in the monomers and complexes and the displacement of the atoms in the nominal C
Au stretching mode for the complexes and respective monomers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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