ARTICLE pubs.acs.org/JPCA
Influence of the Solvent on the Charge Distribution of Anomeric Compounds Antonio Vila, Laura Estevez, and Ricardo A. Mosquera* Departamento de Química Física, Universidade de Vigo, Facultade de Química, 36310-Vigo, Galicia, Spain
bS Supporting Information ABSTRACT: Conformational preferences of methanediol, dimethoxymethane, methanediamine, and fluoromethanol in the presence of solvents of diverse polarity (water, acetone, and chloroform), modeled with the polarizable continuum model, were analyzed within the framework of the Quantum Theory of Atoms in Molecules. The results indicate that the hydrogens bonded to the anomeric carbon experience the largest reorganization of electron density upon conformational change, as was obtained from previous calculations in the gas phase. When the water solvation is simulated by explicit inclusion of water molecules, the electron density reorganization involved in the cluster formation is significantly different for each conformer of methanediol. As a consequence, similar depletions of electron population are displayed by the hydrogens of hydroxyl and methylene groups in the cluster obtained for the most stable conformer of methanediol, with regard to that built for the completely antiperiplanar conformer.
’ INTRODUCTION The term “anomeric effect” was introduced to designate the tendency of an electronegative substituent at C1 of a pyranoid ring to adopt the axial rather than equatorial orientation, in strong contrast with expectations based exclusively on steric grounds. This term was later extended to include open-chain compounds. Thus, the generalized anomeric effect is defined as the preference of the gauche disposition over the anti in segments R-X-A-Y, where A is an element of intermediate electronegativity (like C or Si), Y denotes an atom more electronegative than A, X corresponds to an element possessing at least one lone electron pair, Lp, and R stands for H or an alkyl group.1-3 It has been observed that solvent effects play a decisive role in determining the relative stability of anomeric compounds.1-4 Thus, the energy difference between gauche and anti conformers decreases as the solvent dielectric constant increases. This fact was employed in support of the electrostatic explanation5 of the anomeric effect. According to it, the anti conformation is destabilized because of Lp/A-Y dipole-dipole repulsions that are avoided in the gauche conformations. An alternative explanation, based on the model of charge delocalization, postulates that the stability of the gauche forms is due to the delocalization of one X lone pair into the low-lying antibonding orbital A-Y.6 NBO studies concluded that, depending on the molecular systems and the surrounding medium, both dipole repulsions and charge delocalization factors can contribute, to different extents, to the anomeric effect.1-3,7 In contrast, Mo has recently reported that BLW calculations do not support the popular hyperconjugative model for the anomeric effect and has indicated that an alternative explanation must be introduced.8 Based on the Quantum Theory of Atoms in Molecules (QTAIM) topological analysis,9,10 we have developed another r 2011 American Chemical Society
interpretative model for the anomeric effect, which was initially applied to rationalize the conformational preferences of methanediol and dimethoxymethane.11 This model assigns a relevant role in conformational preferences to the hydrogens attached to the anomeric carbon, Hc. Thus, the gauche stabilization in these two structures is due to the reduction of the electron population experienced by Hc atoms as the number of their gauche arrangements to lone pairs (Hc-C-O-Lp dihedral angles close to 60° or -60°) rises. The electron population lost by these hydrogens flows toward better attractors (oxygen and carbon atomic basins).11 This model also successfully rationalizes the anomeric preferences of other linear and cyclic compounds that contain O-C-O12,13 or N-C-N units.14 The above conclusions were obtained from calculations11-14 carried out in the gas phase. Nevertheless, several QTAIM studies have shown that solvent effects modify electron density distributions significantly.15-18 In view of this, here, we present the results of the QTAIM analysis of the electron densities obtained in the presence of a solvent for diverse anomeric compounds: methanediol (1), dimethoxymethane (2), methanediamine (3), and fluoromethanol (4). The results are compared with those obtained in previous work11-14 in the gas phase. To model solutions, on one hand, we have made use of the polarizable continuum model, PCM,19 considering three solvents whose polarity is significantly different. On the other hand, we have analyzed the effect of explicitly adding solvent (water) molecules in the first coordination sphere of methanediol on the anomeric trends. Both types of calculations can be taken as a first Received: July 31, 2010 Revised: January 17, 2011 Published: February 15, 2011 1964
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Figure 1. Structure, atom numbering, and relative atomic electron populations, ΔN(Ω) (with regard to 1tt and in units of 103 au) for B3LYP/6-311þþG(2d,2p) full optimized methanediol conformers. ΔN(Ω) values in gas phase, ΔgN(Ω) (shown in brackets), and those obtained for solution with PCM single-point calculations on gas-phase geometries, ΔsN(Ω), are ordered by increasing constant ε: chloroform, acetone (in italics), water (underlined).
test to evaluate if the scope of our interpretative model can be extended to solvated compounds or must be modified for them. Moreover, results obtained from those calculations with water can provide additional information about the effect of considering explicit intermolecular interactions with solvent.
’ COMPUTATIONAL DETAILS For comparison purposes, we have retained the nomenclature and atom numbering employed in the gas phase studies reported in our previous papers.11-14 Thus, the conformers of the O-C-O compounds (1 and 2; see Figures 1 and 2, respectively) are labeled using acronyms that are formed by combining t, g, and g0 , which refer to C-O-C-O (1) or H-O-C-O (2) dihedral angles of ∼180°, 60°, or -60°. For molecule 3 (Figure 3), the acronyms indicate the approximate values of the Lp-N-C-N dihedral angles. Finally, t and g refer to the F-C-O-H dihedral angle in compound 4 (Figure 4). It must be stressed that, according to this notation (based on the rules for conformational nomenclature introduced by Klyne and Prelog20), the most favored arrangements for the anomeric effect are represented by g in compounds 1, 2, and 4, and by t in compound 3. Single-point calculations on the fully optimized gas-phase geometries of 1-4 were performed at the B3LYP/6-311þþG(2d,2p) level and solvent (chloroform, acetone, and water) effects were included via the polarizable continuum model, PCM,19,21 as implemented in Gaussian 03,22 using default parameters. This procedure, previously utilized in the literature in calculations involving the anomeric effect,23-25 allows to assess the effects of solvation without interferences due to nuclear reorganization. Discrete water solvation of two conformers of structure 1 (1gg and 1tt) was approached by explicitly adding six water molecules in the DFT calculations. This number of water molecules is the
Figure 2. Structure, atom numbering, and relative atomic electron populations, ΔN(Ω) (with regard to 2tt and in units of 103 au) for B3LYP/6-311þþG(2d,2p) full optimized dimethoxymethane conformers. ΔN(Ω) values in gas phase, ΔgN(Ω) (shown in brackets), and those obtained for solution with PCM single-point calculations on gasphase geometries, ΔsN(Ω), are ordered by increasing constant ε: chloroform, acetone (in italics), water (underlined).
minimum that allows the establishment of intermolecular hydrogen bonds (IHB), involving all of the oxygens, Lps, and hydrogens of methanediol hydroxyls in 1gg and 1tt. The corresponding F(r) functions were obtained from B3LYP/6-311þþG(2d,2p) single-point calculations on the gas-phase-restricted optimized B3LYP/6-31G(d,p) geometries, where the O-C-O-H dihedral angles were kept at the values optimized for isolated 1tt and 1gg conformers. These structures are named, respectively, 1tt 3 6W and 1gg 3 6W (see Figure 5). Numerical integrations over the atomic basins (Ω) were performed with the AIMPAC program series26 to obtain atomic electron populations (N(Ω)) and energies (E(Ω)). As Pusual, the accuracy of these calculations was checked P by comparing N(Ω) to the total electron populations and E(Ω) to the total electron molecular energy. We have found that the total electron populations and electronic molecular energies are recovered within 0.001 au and 1 kJ mol-1, respectively. Also, no atom displayed L(Ω)9,10 absolute values larger than 10-3 au.
’ RESULTS AND DISCUSSION PCM Calculations. First of all, we observe that solvation has an important effect on the conformational equilibrium of the anomeric compounds considered in this work (see Table 1). In the O-C-O-containing compounds and fluoromethanol, the solvent reduces the preference for the most-favored conformer, whereas the opposite trend is found in methanediamine. These 1965
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Figure 5. Hexahydrated structures here considered for 1tt and 1gg showing IHB and water nomenclature.
Table 1. Relative Energies (kJ mol-1) for the Conformers of Methanediol (1), Dimethoxymethane (2), Methanediamine (3), and Fluoromethanol (4) In Vacuo and in Solution Figure 3. Structure, atom numbering, and relative atomic electron populations, ΔN(Ω) (with regard to 3gg and in units of 103 au) for B3LYP/6-311þþG(2d,2p) full optimized methanediamine conformers. ΔN(Ω) values in gas phase, ΔgN(Ω) (shown in brackets), and those obtained for solution with PCM single-point calculations on gasphase geometries, ΔsN(Ω), are ordered by increasing constant ε: chloroform, acetone (in italics), water (underlined).
Relative Energy (kJ mol-1)
a
Figure 4. Structure, atom numbering, and relative atomic electron populations, ΔN(Ω) (with regard to 4t and in units of 103 au) for B3LYP/6-311þþG(2d,2p) full optimized fluoromethanol conformers. ΔN(Ω) values in gas phase, ΔgN(Ω) (shown in brackets), and those obtained for solution with PCM single-point calculations on gas-phase geometries, ΔsN(Ω), are ordered by increasing constant ε: chloroform, acetone (in italics), water (underlined).
results agree with those of Montagnani and Tomasi.27 Also, similar attenuations of the anomeric effect in the presence of a solvent were reported for structure 2 and related systems such as 2-methoxytetrahydropyran.28,29 The reduction of the energy gap between 1gg and 1gg0 is also significant for all the solvents here considered. In contrast, the solvent enlarges the stabilization of the methanediamine conformers with favorable anomeric interactions: 3tt and 3tg, as found by Carballeira and Perez-Juste in aqueous solution at the HF level.30 The reason why solvents give rise to opposite effects in O-C-O- and N-C-N-containing anomeric compounds is one of the points explained by our present QTAIM analysis (see below). Finally, for fluoromethanol, the influence of the solvent is moderate, yielding variations in the relative stability of conformers that are below 1 kcal mol-1, with regard to the gas phase. For this compound, we did not found any previous study on the effect of the solvent in the conformational equilibrium.
in vacuoa
chloroform
acetone
water
1tt
32.1
24.4
20.6
19.6
1tg
14.5
12.0
10.8
10.4
1gg0
11.2
5.7
2.9
1.9
1gg
0.0
0.0
0.0
0.0
2tt
23.3
17.5
15.1
14.5
2tg 2gg
11.0 0.0
8.9 0.0
8.2 0.0
7.8 0.0
3gg
4.6
6.8
8.0
8.4
3tg
2.5
3.7
4.4
4.7
3tt
0.0
0.0
0.0
0.0
4t
20.6
18.5
17.4
17.2
4g
0.0
0.0
0.0
0.0
Data taken from refs 11 and 14.
We now concentrate on the variations of atomic electron populations. As noted previously,11 in the gas phase, the largest absolute ΔN(Ω) values displayed in compound 1 correspond to the methylenic hydrogens, Hc, that, in a given conformer, display more gauche orientations to Lps than in 1tt (H6 and H7 in 1gg, H6 in 1gg0 , and H7 in 1tg; see Figure 1). All of them are negative, reflecting that electron density is expelled from each Hc atomic basin as its number of Lp-O-C-Hc gauche arrangements increases. This trend is kept in the calculations performed with the three solvents, but the corresponding ΔN(Hc) values are less negative than in the gas phase. Note that the larger the constant ε, the smaller the electron density depletion experienced by these Hc basins (see Figure 1). In the gas phase, we also observe important increases for N(C) whenever the electron density of Hc basins is depleted with regard to that in 1tt. This trend is also kept in solution and, in some cases, ΔN(C) even remains nearly unchanged by the solvent (within 2 10-3 au). The electron populations of oxygens in 1 are also significantly modified in the gas phase by conformational changes. In contrast, this is not observed in solution, where ΔN(O) values for 1tt f 1gg and 1tt f 1gg0 interconversions are negligible, whereas those corresponding to 1tt f 1tg are ∼50% lower than those in the gas phase. 1966
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Table 2. ΔΔN(Ω) Values (with Regard to the Corresponding Reference Conformera) for the Atoms in Central Methyleneb ΔΔN(Ω) Value Cc chlorof.
acetone
Hcg water
chlorof.
chlorof.
acetone
water
chlorof.
acetone
water
16
17
-1
-2
-1
-10, -6
-14, -9
-15, -10
12
13
-14
-19
-21
25
27
-5
-5
-7
6
9
10
-9, -5
-13, -9
-14, -10
-12
-18
-19
3, 0
3, -1
4, 0
3
3
4
-10
-12
-13
0
0
1
11
1gg
0
-1
-1
8
1gg0
0
-1
-2
17
2tgc
-1
-1
-1
1tg
O/N
water
c
acetone
Hc=
2gg
0
-1
-1
4
7
8
3tgc
-2
0
-2
5
7
8
3tt
-1
-1
-2
7
11
12
4g
1
2
1
8
12
13
0
0
0
0
1
1
-2
-3
-3
a
Reference conformers are 1tt, 2tt, 3gg, and 4t. b Cc denotes the carbon, Hcg denotes the hydrogens with more gauche arrangements to oxygen Lps in the actual conformer than in the reference, and Hcd denotes those whose number of gauche arrangements in the conformer do not change, with regard to the reference. c The first and second values correspond, respectively, to the O/N atom contained in the main dihedral angle in the antiperiplanar and gauche arrangements.
Two features can be invoked to explain the above-described evolution of F(r) in conformational interconversions that take place in the presence of solvents: (1) The electron densities computed with PCM are more polarized than those obtained for the gas phase (see Table S3 in the Supporting Information). Thus, in general, solvation depletes N(H) values while the N(O) ones are reinforced. As a consequence, it is not surprising that extracting the electron density from hydrogens, or transferring it into oxygens, is more difficult in solution than in the gas phase. In contrast, the carbon atom displays very similar behaviors in the gas and solution phases. (2) Electron-electron repulsions are attenuated by increased ε constants. Therefore, according to our electron-density-based model for the anomeric effect (which associates these repulsions to F(r) reorganization along the conformational changes), smaller ΔN(Ω) values should be displayed when ε increases, as is observed. Differences between relative atomic populations obtained in solution and in the gas phase (ΔΔN(Ω) = ΔsN(Ω) - ΔgN(Ω)) (see Table 2) clarify this discussion. We observe that Hcs with more gauche interactions to Lps in a certain conformer than in the reference compound (1tt for methanediol), labeled as Hcg hereafter, display clearly positive ΔΔN(Hcg) values, indicating that the amount of F(r) taken from them is reduced by solvent effects, as expected from both features indicated above. In contrast, Hcs with the same number of gauche interactions to Lps than in the reference conformer, Hcd, as well as methylenic carbons, Cc (which are never affected by the features described above) show negligible ΔΔN(Ω) values. It can be observed that this behavior is followed in the four compounds that are considered here (Table 2). The larger ΔΔN(Hcg) values observed for 1gg0 are explainable by the larger increase of gauche interactions experienced by H6 on going from 1tt to 1gg0 . Overall, the largest variations in molecule 1 occur for Hcg, in the presence of one of the solvents or in the gas phase. The extent of the redistribution of atomic electron populations in the molecule is reduced (as well as the anomeric preference
measured by the relatives energies presented in Table 1) as the dielectric constant increases. Moreover, as the solvent reinforces the polarity of the solute, N(O) increases, making the oxygen basins less appropriate to receive the electron density expelled from Hc atoms, which then must be distributed among Cc and hydroxylic hydrogens (see Figure 1). The lower ability of oxygens of solvated methanediol to accept electron density is at the origin for the reduction of its anomeric preference in solution. In fact, the largest variation due to solvation among the relative QTAIM energies in 1gg, with regard to 1tt (ΔE(Ω)), corresponds to oxygen atoms (see Table 3). Thus, ΔE(O) is negative in the gas phase and becomes substantially positive upon solvation. In contrast, solvation makes ΔE(Ω) negative for hydroxylic hydrogens. The relative stabilization of these hydrogens does not compensate for the destabilization of oxygens. Thus, the relative energy of 1tt decreases in solution, because transferring the electron density to hydroxylic hydrogens is much less stabilizing than sending it to oxygens. We also infer, from Table 3, that the methylene group in 1gg is always (both in the gas phase or in solution) more stable than in 1tt. Solvation favors this trend but, in contrast, makes it such that hydroxyls groups in 1tt are more stable than those in 1gg. The ΔΔN(Ω) (ΔsN(Ω) - ΔgN(Ω)) values shown in Table 2 indicate that the other O-C-O-containing structure, dimethoxymethane, 2 (Figure 2), behaves basically like methanediol. We observe again, both in solution media and in the gas phase, a large loss of atomic electron population in Hc basins when the number of gauche interactions to Lps increase along a certain conformational change (such as 2tt f 2tg or 2tt f2gg). In isolated dimethoxymethane, most of this electron density is transferred to oxygens and methylenic carbon, C1, and only a small amount goes to the methyl groups, whose ΔgN(Ω) are mainly related to the increased repulsions experienced by certain hydrogens (H12 in 2tg, H10 and H12 in 2tt), which approach oxygen Lps, with regard to 2tt.11 In solution, the transference from Hcg diminishes, but the ability of oxygens to receive electron density is even more depleted. As a consequence, more electron density arrives at methyls as ε increases. Therefore, ΔsN(Ω) values in the methyl groups are more ruled by the electron density transferred from Hcg than by electron-electron repulsions between methyl hydrogens and oxygen Lps. ΔE(Ω) values (Table 3) indicate 1967
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Table 3. ΔE(Ω) Values (with Regard to the Corresponding Reference Conformera) for Conformers with One C2 Axisb ΔE(Ω) (kJ mol-1) for 1gg-1tt ΔE(C1)
ΔE(H6)
ΔE(O2)
ΔE(H5)
ΔE(-CH2-)
ΔE(-OH) -6
in vacuo
-70
25
-7
1
-20
chloroform
-66
13
16
-8
-40
8
acetone
-62
12
21
-12
-38
9
water
-60
11
23
-13
-38
10
ΔE(Ω) (kJ mol-1) for 2gg-2tt
in vacuo chloroform
ΔE(C1)
ΔE(H6)
ΔE(O2)
ΔE(CH3)c
ΔE(-CH2-)
ΔE(-OCH3)
-70 -69
18 14
43 57
-37 -45
-34 -41
6 12
acetone
-67
11
63
-47
-45
16
water
-66
10
64
-47
-46
17
ΔE(Ω) (kJ mol-1) for 3tt-3gg ΔE(C1)
ΔE(H2)
ΔE(N4)
ΔE(H5) þ ΔE(H6)
ΔE(-CH2-)
ΔE(-NH2)
in vacuo
-24
30
3
-23
36
-20
chloroform acetone
-25 -24
18 17
2 2
-11 -11
11 10
-9 -9
water
-24
16
2
-10
8
-8
ΔE(Ω) (kJ mol-1) for 4g-4t
a
ΔE(C2)
ΔE(H5) þ ΔE(H4)
ΔE(F1)
ΔE(-CH2-)
in vacuo
-33
25
53
-8
-65
chloroform
-45
19
73
-26
-66
acetone water
-45 -45
16 14
79 80
-29 -31
-67 -67
Reference conformers are 1tt, 2tt, 3gg, and 4t. b See figures for atom numbering. c Methyl group formed by C5, H11, H12, and H13.
Table 4. Main Properties of Water/Methanediol and Water/ Water IHBs in Hexahydrated Structures Considered for Methanediola 1tt
Table 5. B3LYP/6-311þþG(2d,2p)//B3LYP/6-31G(d,p) Molecular Energies (E) and Main Geometrical Features for Optimizeda Hexahydrated Clusters of Methanediol and Their Corresponding Isolated Conformers
1gg 1tt 3 6W
R (Å) θ (deg) 102Fb (au) R (Å) θ (deg) 102Fb (au) O-H 3 3 3 OW O 3 3 3 HW-OW(l)
E (au)
1gg 3 6W
1tt
1gg
-649.57639 -649.87849 -191.01424b -191.02647b
1.778 147.2
3.85
1.621 166.1
5.74
2.203 126.3
1.67
2.051 144.1
2.11
C-O
1.411
1.414
1.406
1.409
O 3 3 3 HW-OW(s) 1.937 158.4 OW2 3 3 3 HW1-OW1 1.786 156.7 OW6 3 3 3 HW2-OW2 1.890 153.3
2.69
1.817 165.6
3.35
O-H
0.989
1.011
0.961
0.963
3.74 2.87
1.974 142.2
2.49
1.786 160.9
3.73
104.4 108.8
113.1 109.0
OW6 3 3 3 HW1-OW1
Bond Lengths (Å)
Bond Angles (deg)
R = bond length, θ = bond angles, and Fb = the electron density at the BCP. a
ΔE(-OH)
that, as obtained for 1, the depletion of anomeric stabilization is due to the destabilization of polar groups (-OCH3), which is not compensated by the increased stabilization of the central methylene. In contrast with O-C-O-containing compounds, anomeric stabilizations are significantly enhanced in methanediamine, 3, in the presence of the solvent media here considered (see Table 1). As previously reported for the isolated compound,14 the most relevant decreases in N(Ω) (with regard to 3gg) also take place
O-C-O C-O-H
105.7 105.9
113.0 108.3
H-O-H (W1)
105.1
106.8
H-O-H (W2)
100.3
105.0
H-O-H (W3)
105.0
104.8
H-O-C-O dihedral angles were fixed at their gas-phase optimized values. b Molecular energy after B3LYP/6-311þþG(2d,2p) full geometry optimization. a
for the Hcg atoms (H2 and H3 in 3tt and H2 in 3tg) in solution. However, these decreases are less marked as ε increases. The increase in atomic population for the aminic hydrogens H6 and H9 that leave the parallel disposition to nitrogen Lps (e.g., 1968
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Table 6. QTAIM Atomic Electron Populations (N(Ω)) for Atoms of 1, Water Molecules, and Charge Displayed by 1 (q1) in the Conformers Here Studied for Hexahydrateda and Isolatedb Methanediol
a
P
W1N(Ω)
P
W2N(Ω)
P
N(C) (au)
N(Hc) (au)
N(O) (au)
N(H) (au)
1tt 3 6W
5.016
0.987
9.125
0.387
10.001
9.994
9.998
-14
1gg 3 6W
5.028
0.974
9.153
0.373
9.996
9.996
9.994
-28
1tt
4.984
1.014
9.048
0.446
1gg
5.003
0.985
9.068
0.446
(au)
(au)
W3N(Ω)
(au)
q1 ( 103 au)
Obtained from B3LYP/6-311þþG(2d,2p)//B3LYP/6-31G(d,p) electron densities. b Obtained from B3LYP/6-311þþG(2d,2p) electron densities.
N7-H9 leaves the parallel disposition to N4-Lp both in 3tt and 3tg) is also considerably less marked in the solvent media. Both facts can be related to the depletion of electron-electron repulsions as ε increases. Moreover, variations of atomic electron populations (due to conformational change) in solution, ΔsN(Ω), are more similar to those of the gas phase, ΔgN(Ω), than those in 1 and 2 (see Figure 3). This is particularly noticeable for N atoms (in 3) compared to oxygen atoms (in 1 and 2), as shown in Table 2 by ΔΔN(Ω) values. This means that 3 is less polarized by solvents and its nitrogen atoms do not lose the ability to receive electron density. In contrast, this ability is slightly enhanced by solvation. Therefore, the transference of electron density from hydrogens to heavy atoms upon internal rotation from g to t arrangements is increased by solvation, enhancing the stability of the latter. ΔE(Ω) values (Table 3) indicate that, in this case, the increase in the anomeric stabilization comes from the balance between a lower stabilization of -NH2 groups and a lower destabilization of central methylene. Finally, for fluoromethanol, 4, the Hcg atom in this compound (H5), also plays the decisive role in the electron density redistribution, both in the gas phase and in solution (Figure 4). This hydrogen still displays the most negative ΔN(Ω) value upon going from 4t to 4g, and its absolute value becomes smaller as the polarity of the solvent increases. Thus, in water solution, ΔN(H5) is half of its value in the gas phase. As previously commented, PCM enlarges N(O) values, with regard to the gas phase. This makes the transference of electron density driven by even smaller electron repulsions (diminished as ε increases) more difficult. As a consequence, ΔN(O) values are significantly depleted with PCM. ΔN(F1) is very small, because F electronegativity is high enough to make the ability to receive more electron density driven by conformational changes or solvation very small. According to ΔE(Ω) values (Table 3) the depletion in the anomeric stabilization upon solvation is originated by destabilization of the F atom in solution. Hexahydrated Methanediol: 1gg 3 6W and 1tt 3 6W. In this section, we are only concerned with the possibility that explicit IHBs with solvent molecules could modify the anomeric effect. Thus, the effect of adding explicitly water molecules was tested in 1gg and 1tt conformers of methanediol, restricting our study to one example with six water molecules, 1gg 3 6W and 1tt 3 6W, with the same number and type of IHBs. Figure 5 depicts the corresponding bond paths and critical points (CPs) associated to the intermolecular interactions within each cluster, 1gg 3 6W and 1tt 3 6W. The relative energy of 1gg 3 6W cluster, with regard to 1tt 3 6W (-793 kJ mol-1), cannot be related to the anomeric effect. Most of it is due to the different distortions experienced by methanediol to establish IHBs with water molecules. The different strength of the IHBs formed in each cluster (inferred from data listed in Table 4) is
Table 7. Relative QTAIM Atomic Electron Populations for Conformer gg, with Regard to tt, for Hexahydrated and Isolated 1 (Δ1 3 6WN(Ω) and Δ1N(Ω)), and Variations of Atomic Electron Population upon Cluster Formation for 1tt N(Ω) and Δcluster N(Ω)) and 1gg Conformers (Δcluster tt gg P
C
Hc
O
H
Δ1 3 6WN(Ω) ( 103 au)
12
-13
28
-14
Δ1N(Ω) ( 103 au)
19
-29
20
0
0
N(Ω) ( 103 au) Δcluster gg
25
-11
85
-73
28
N(Ω) ( 103 au) Δcluster tt
32
-27
77
-59
14
f
1f
14
also included in this quantity. Table 5 lists the main geometrical features of both clusters. Despite geometry distortions due to the cluster formation that are not equivalent for both structures, we highlight that cluster formation does not affect the main geometry hallmarks of the anomeric effect significantly. Regarding the atomic electron populations in hexahydrated clusters, we observe the methanediol monomer displays small negative charges, q1, in both clusters (see Table 6). This is due to the electron density transferred from water molecules, larger in 1gg 3 6W. All of them, excluding W1 and W4 in 1tt 3 6W, are positively charged. The fact that all water molecules are involved at least in two IHBs prevents the assignment of electron density transferences to each IHB. Moreover, despite the fact that N(Ω) values vary significantly from isolated to hexahydrated methanediol, the reduction in the electron populations of Hc atoms in 1gg 3 6W, with regard to 1tt 3 6W (see Δ1.6WN(Ω) values in Table 7), is almost the same as that in water solvation using PCM calculations (Figure 1). In the same vein, N(C) increases when going from 1tt 3 6W to 1gg 3 6W to a similar extent as that with PCM modeled water solution. In contrast, the largest variations of Δ1.6WN(Ω) values correspond to oxygens, which are almost unaffected according to PCM calculations. Moreover, hydroxyl hydrogens lose electron density when the conformational change takes place in the hexahydrated cluster, whereas they receive it in PCM modeled solutions. The reason for this different behavior is clear when we look at the difference between N(Ω) values for the same conformer in the cluster and an isolated molecule N(Ω) and Δcluster N(Ω) values (Table 7)). These values (Δcluster gg tt indicate how water molecules in the cluster transfer electron density and polarize methanediol. We find that variations due to cluster formation are (i) larger than global charge transference for almost all atoms (C and Hc in 1gg 3 6W are the exceptions), revealing that cluster formation is mainly governed by polarization; (ii) larger for the hydroxyl atoms; and (iii) significantly different from one conformer to another, with hydroxyls more affected in 1gg 3 6W. As a consequence of the latter observation, the differences between Δ1 3 6WN(Ω) values and those obtained for PCM modeled solutions may be ascribed to the different 1969
dx.doi.org/10.1021/jp1072022 |J. Phys. Chem. A 2011, 115, 1964–1970
The Journal of Physical Chemistry A polarization and charge transference experienced by each conformer upon cluster formation. Overall, when water molecules are included explicitly, our QTAIM-based interpretation of the anomeric effect must be combined with the different effects provided by the cluster formation.
’ CONCLUSIONS Quantum Theory of Atoms in Molecules (QTAIM) analysis of the B3LYP/6-311þþG(2d,2p) electron densities obtained via the polarizable continuum model, for solvated (with water, acetone and chloroform) methanediol, dimethoxymethane, methanediamine, and fluoromethanol, lead us to the following major conclusions: (1) The fundamental role of hydrogen atoms in the atomic population reorganization that begins the anomeric stabilization found in the gas phase holds for the corresponding solvated electron densities. Thus, the number of gauche interactions of these hydrogens with lone electron pairs (Lps) and the parallel-like alignments with them are the main driving force for interatomic transferences of electron density. (2) The reorganization of electron density associated with conformational changes decreases when the polarity of the solvent increases. For methanediol, the relevant role of gauche interactions with the oxygen Lps is also found when the water solvation is simulated by explicitly adding water molecules. Nevertheless, the electronic effects provoked by cluster formation with solvent molecules may differ in both conformers more than the variations originated by the in vacuum conformational change. ’ ASSOCIATED CONTENT Supporting Information. Complete tables of ΔΔN(Ω) values for compounds 1-4. (PDF) This information is available free of charge via the Internet at http://pubs.acs.org.
bS
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank allocation of computational resources to Centro de Supercomputacion de Galicia and financial support from Xunta de Galicia (Project Nos. INCITE08PXIB314224PR and INCITE09E1R3141091ES) and Spanish MICINN (Project No. CTQ2006-15500/BQU).
ARTICLE
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