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Electronic Interpretation of Conformational Preferences in Benzyl Derivatives and Related Compounds Antonio Vila,† Mercedes García Bugarín,‡ and Ricardo A. Mosquera*,† †
Departamento de Química Física and ‡Departamento de Química Inorganica, Facultade de Química, Universidade de Vigo, Lagoas-Marcosende s/n, 36310-Vigo, Galicia, Spain
bS Supporting Information ABSTRACT: Quantum Theory of Atoms in Molecules (QTAIM) analysis on B3LYP/ 6-311++G(2d,2p) 6d electron densities of five benzyl derivatives (C6H5CH2X; X = F, Cl, OH, SH, NH2) and seven related fluorides of furan, pyrrole, and naphthalene indicates that the preference for perpendicular or gauche conformation exhibited by these compounds is related to the diminution of the steric repulsion between the heteroatom at the substituent and the closest hydrogen in the ring. The electron density reorganization can be satisfactorily explained on the basis of these repulsive interactions, while no evidence of larger hyperconjugative delocalization is observed in the preferred conformations.
’ INTRODUCTION The anomeric effect, first uncovered as an anomaly in the conformational preferences of sugars, was shown to play a relevant role in the conformational preference of diverse compounds.13 This effect establishes the gauche preference of the RYCZ unit, where Y is an atom bearing at least one lone pair, Lp, of electrons and Z is an electronegative atom. According to the socalled hyperconjugative model, the stabilization is believed to arise from the delocalization of one unshared lone pair at Y and the antibonding orbital CZ (nYfσ*CZ delocalization).4 By means of spectral analyses and ab initio methods, Penner et al.5 determined the conformational preferences of some benzylX derivatives with the aim of establishing whether stabilizing interactions enhance the conformational preference for the perpendicular structure (Figure 1). Except for X = F, compounds studied [X = Cl, SH, SCH3, S(O)CH3] adopted mainly the conformation where the CZ bond (Z being the electronegative atom bonded to the CH2 group) is perpendicular to the plane of the benzene ring. The authors proposed therefore the existence of a “benzylic anomeric effect” through π f σ*CZ hyperconjugation and that its magnitude was S(O)Me > Cl > SH, SMe > F. Later on, several papers, reported that benzylfluoride was not an exception to this rule and also preferred the perpendicular arrangement for CCCZ units.610 The purpose of the present work is to analyze the conformational preferences of diverse benzyl derivatives (and some related compounds) within the framework of the Quantum Theory of Atoms in Molecules1113 (QTAIM). We aim to get insight into the reasons that lead to the stabilization of the perpendicular conformations. This paper is part of a series devoted to the study of the anomeric effect and related phenomena within the QTAIM approach.1417 Our previous work in this series has shown that the anomeric conformational preferences of OCO1416 and NCN17 containing compounds can be satisfactorily explained r 2011 American Chemical Society
on the basis of the electronic population reorganization due to steric interactions. We also evidenced that the evolution of the QTAIM atomic populations and delocalization indexes could not be accommodated within the hypothesis of the stereoelectronic delocalization.4 Thus, we conclude the anomeric effect is not due to hyperconjugative delocalizations but to steric interactions. In the same vein, using valence bond calculations for various anomeric compounds, Yirong Mo has provided strong evidence that hyperconjugation interactions are not responsible for the anomeric effect and that it is better interpreted with electrostatic interactions.18
’ COMPUTATIONAL DETAILS Full geometry and/or symmetry restricted optimizations for the conformations under investigation were carried out at the B3LYP/6-311++G(2d,2p) 6d level using the Gaussian 03 program.19 The numerical integrations over the atomic basins (Ω) were executed with the AIMPAC program series20 to obtain the reported atomic populations, N(Ω), and energies, E(Ω). Splitting of N(Ω) values into σ and π components is precluded by the lack of the σh plane in perpendicular conformations. The delocalization indexes among every pair {Ω1, Ω2} of atomic basins, δ(Ω1, Ω2), were computed as defined within the context of QTAIM21 by means of a program developed in our laboratory that reads the values of the overlap matrix for every MO from the AIMPAC output. Moreover, the calculation of the corresponding QTAIM n-center delocalization indices, Δn,22 is implemented in the same program. Special Issue: Richard F. W. Bader Festschrift Received: May 31, 2011 Revised: September 11, 2011 Published: September 22, 2011 13088
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The Journal of Physical Chemistry A Taking into account that this work deals with a small difference in energy between conformers, we ensured the QTAIM numerical procedure was carried out with an acceptable accuracy. Thus, summation of N(Ω) and E(Ω) values for each conformation reproduced total electron populations and electronic molecular energies within 2 104 au and 0.6 kJ mol1, respectively. No atom was integrated with absolute values of L(Ω) function1113 larger than 9 104 au. The minima of the Laplacian of the electron density, r2F(r), which represent maxima of local charge concentration, have also been located in this work. Some of them appear in bonding regions and are related to the position of the corresponding bonding electron pairs. Other minima of r2F(r) are found in nonbonded regions, which are known as nonbonded charge concentrations (NBCCs). They are particularly important for our discussion as they are found in the expected positions of the Lp's in the Lewis theory.1113 The search for critical points in the Laplacian of the electron density was performed using AIMPAC.20 The programs and methods indicated above were applied to study 12 molecules including diverse aromatic rings (benzene, furan, pyrrole, and naphthalene) and substituents (CH2F, CH2Cl, CH2OH, CH2SH, CH2NH2). While all the substituents indicated were combined with benzene (molecules 15), for furan, pyrrole, and naphthalene we limited our study to CH2F, considering all possible position isomers (molecules 612). For all the molecules, we have performed a detailed conformational analysis, including computation of energy profiles for internal rotations around the dihedral angles defining conformations. They are named according to the approximate values of one or two dihedral angles depending on the molecule (s, g, p, g0 , and t representing, respectively, values close to 0, 60, 90, 60, and 180°). Thus, in all molecules, we indicate first the CCC*Z dihedral angle, j, where C* represents the carbon attached to
Figure 1. Stereoelectronic explanation of conformational preferences of benzylX compounds.
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phenyl, furan, pyrrole, or naphthalene and Z is the heteroatom in the substituent (F, Cl, O, S, or N). When necessary, the description is complemented indicating the disposition of CC* ZH (when Z = O, S) or CC*ZLp (when Z = N), denoted as χ. Stationary points were characterized as conformers or transition states on the basis of their vibrational frequencies.
’ RESULTS AND DISCUSSION Benzylfluoride. In agreement with previous computational studies610 of the two stationary points obtained for benzyl fluoride (Figures 2 and 3) at the B3LYP/6-311++G(2d,2p) level, only the perpendicular conformation was confirmed as the true minimum by the vibrational frequency analysis (all frequencies being real). Thus, at the current computational level, the perpendicular conformation (1p) is 1.9 kJ mol1 more stable than the planar one (1s), which is a transition state for the internal rotation around the C1C* bond (Figure 2). This value is in close agreement with those reported in ref 9 at several computational levels. Looking at optimized geometries (Table 1 and Supporting Information), we realize that some anomeric structural trends are followed: CC* shrinks in 1p with regard to 1s, while the opposite is true for C*F. Nevertheless, the C1C*F angle does not widen (it closes by 1.1°) on going from 1s to 1p as the stereoelectronic model predicts. In contrast, we notice that C2C1C* closes by 1.2° in 1p, in line with the important relaxation experienced by the interactions between H2 and F. Table 2 lists variations of averaged distances from nuclei to fluorine Lp's represented by the maxima of NBCCs around F, ΔR1p1s (Ω, Lp). Data in Table 2 show that the largest variation in atomic population corresponds to H2. This is the closest hydrogen to
Figure 3. Structure and atom numbering of planar (1s) and orthogonal (1p) benzyl fluoride. ΔE (Ω) values (in kJ mol1) with regard to E(Ω) ones in 1s are also shown for 1p.
Figure 2. B3LYP/6-311++G(2d,2p) 6d energy profiles for internal rotation around the C1C* bond in benzyl fluoride (1) and benzyl chloride (2) (a) and for benzyl alcohol (3) when χ is in g and t arrangements (b). Relative energies (ΔE) in kJ mol1 and j in degrees. 13089
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Table 1. Main B3LYP/6-311++G(2d,2p) 6d Optimized Dihedral Angles (degrees) for Benzyl Derivativesa i
j
b
1
0.0
-
370.91950
1p 2sb
0 1
90.0 0.0
-
370.92022 731.27519
2p
0
90.0
-
3stb,d
1
0.0
3st
0
3gg
0
1s
χ
E
Table 2. QTAIM ΔN(Ω) Values (in au) for p in 1 and 2 with Regard to s Conformation and Relative Average Distances to Fluorine Lp's (ΔR1(Ω,Lp)) in 1 (in Å) 103ΔN1p1s(Ω)
ΔR1p1s (Ω,Lp)
103ΔN2p2s(Ω)
C1
19
0.043
18
C2
2
0.444
0
731.27835
C3
1
0.266
0
180.0
346.89226
C4
3
0.100
3
14.6
173.2
346.89228
53.7
54.5
346.89404
C5 C6
1 11
0.415 0.498
0 10
3pg
0
90.0
49.1
346.89335
H2
27
0.909
21
3ptd
0
90.0
0.0
346.89196
H3
2
0.456
1
4stb,d 4gg
1 0
0.0 74.4
180.0 55.9
669.86490 669.86949
H4
3
0.107
3
H5
3
0.578
2
4ptd
0
90.0
180.0
669.86865
H6
11
0.779
10
5st
0
0.0
180.0
327.01942
C*
13
0.013
5
5gg0
0
42.0
54.0c
327.02026
5pt
0
90.0
180.0
327.02053
H* H*
9 9
0.017 0.019
6 6
a
Total molecular electronic energies, E, in au, and number of imaginary vibrational frequencies, i, are also shown. b Transition state. c Obtained from bisecting C1C*NH dihedral angles. d Optimized under Cs restrictions.
the fluorine atom in 1s and experiences an important reduction of F 3 3 3 H repulsions on going from 1s to 1p (F 3 3 3 H2 interatomic distance being 2.383 and 3.307 Å, respectively). In the same vein, we observe noticeable diminutions of N(C1), N(C6), and N(H6), which can also be related to the decrease of their nonbonding distances to F in the perpendicular conformation. It can be said that atomic electron populations decrease or increase in the conformational change depending on what the average of distances from the nuclei to the three fluorine NBCCs does. In fact, positive ΔN1p1s(Ω) values, except for H3, are accompanied by an average departure from fluorine Lp's (Table 2). Also, there is an approach of atomic nuclei toward fluorine Lp's in the perpendicular conformation. This is in line with the observation of Gillespie et al.,23 who have shown there is a conformational preference for the most compact structure. It is noticeable that N(F) diminishes on going from 1s to 1p in spite of reducing its repulsions with H2. Nevertheless, this new arrangement results in higher repulsions with the π system of benzene impelling 0.010 au of electron density out of the fluorine basin. Thus, the redistribution of F electron density from 1s to 1p is mainly ruled by the balance between electron repulsions between F and H2 and F and the phenyl ring. Overall, the benzene ring electron population is 0.021 au larger in 1p than in 1s. Is this due to an increase of the ring π electron delocalization over σ*C*F? To answer this question, we analyze the variations experienced by δ(Ω1,Ω2) values. They also confirm the important role of the H2 3 3 3 F interaction in determining the conformational stabilities. As can be seen in Table 3, the largest variation corresponds to δ(H2,F). The important role of the fluorine atom in the variation of the ring carbon atomic populations is also evident in Tables 2 and 3. Nevertheless, some positive values of Δδ(Ω1,Ω2), like those of F with C1, C4, and C6, and the fact that six center delocalization indices, Δ6, computed for the phenyl ring in both conformations are slightly larger in 1-s (less than 0.3 104 au) could still be invoked to support the stereoelectronic model. Moreover,
Z
10
19
delocalized atomic electron populations, Nd(Ω) (defined by eq 1, in contrast to localized atomic electron population, Nl(Ω), defined by eq 2) increase for the atoms of the methylene group of the substituent (Table 4) resulting in an overall increase for CH2F (0.014 au). This is not counterbalanced by a similar depletion within the benzene ring, where ΣΔNd(Ω) is only 0.004 au. Now, we will see that, far from supporting hyperconjugative delocalizations, the analysis of Nd(Ω) values indicates they can only play a secondary role in 1p stabilization. Nd(Ω) includes not only 1,3 and further delocalizations but also the electron population shared by atoms attached by Lewis bonds. Thus, variations in bond strength modify Nd(Ω) values. This fact becomes evident when δ(Ω1,Ω2) values are combined to obtain the electron populations shared with atoms attached by bonds, Nbs(Ω), and that shared because of delocalization between nonbonded atoms, Nnb(Ω). We have also considered that both quantities can be computed when Ω and Ω0 belong to the phenyl ring, Nbsr(Ω) and Nnbr(Ω), to the substituent, Nbss(Ω) and Nnbs(Ω), or each of them is in a different region, Nbsi(Ω) and Nnbi(Ω). Table 4 lists the variations of all these quantities for the diverse atoms on moving from 1s to 1p. N d ðΩÞ ¼
∑ 0
δðΩ, Ω0 Þ=2
ð1Þ
Ω 6¼ Ω
N 1 ðΩÞ ¼ δðΩ, ΩÞ
ð2Þ
Looking at Table 4, we realize that most of the increase in delocalized electron population experienced by the CH2F group corresponds to larger electron sharing between bonded atoms, so it cannot be due to any kind of πfσ*C*-F transference. In this context, although F(rc) for C*F is lower (by 7.4 103 au) in 1p than in 1s and the bond lengthens (Supporting Information), δ(C*,F) remains unchanged, pointing to the fact that CF has essentially the same bond order in 1p than in 1s. In contrast, the remaining bonds in the group (C*H* and C1C*) become stronger. Overall, only 0.002 au of those 0.014 correspond to the ΔNnbi(Ω) term of delocalized population, and at the same time, the delocalization within the ring is reduced in 0.003 au. Therefore, we cannot rule out that a certain π f σ*C*-F 13090
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Table 3. Relative B3LYP/6-311++G(2d,2p) Values of δ Indexes in 1p and 2p (in Parentheses) with Regard to Those in the Corresponding s Conformation in au and Multiplied by 103a C1
C2
C1
19(22)
C2 C4
14(26)
C3
C4
C6
H2
H6
H*
C5
7(8)
07(8)
10(9)
H2
12(18)
26(28) 9(8)
H6 6(21)
9(7)
H* H* Z a
H*
12(17) 10(14)
C6
C*
C*
6(5) 15(13)
14(19)
7(11)
9(15)
Values in the range (5,+5) 3 103 au are not shown.
8(7) 29(50)
6(5) (37)
Table 4. Relative Values (for 1p with Regard to 1s) of the Diverse Terms of Electron Density Splitting through Delocalization Index Analysis (See Text)a ΔNl(Ω)
ΔNbsr(Ω)
ΔNbsi(Ω)
ΔNbss(Ω)
ΔNnbr(Ω)
ΔNnbi(Ω)
C3
2
2
1
2
0
C2
2
3
5
1
3
2
6
C1
19
0
7
C6
10
1
5
0
4
C5 C4
3 3
2 1
0 3
1 0
0 1
H3
2
0
0
0
0
H2
26
2
6
4
8 2
3
H6
9
1
2
2
H5
2
1
0
0
0
H4
2
1
1
1
0
C*
3
16
H* H*
6 6
3 3
F C6H5 CH2F total a
ΔNd(Ω)
2
8
18
4
8
14
9
9
3
5
8
0
2 2
0 1
0 0
7
0
0 6 6
ΔNnbs(Ω)
3 3
9
6
9
3
2 2
0
3
4
0
All values in au and multiplied by 103.
transference may take place in certain conformations along the internal rotation around the C1C* bond, but it can be stated that it is not the main effect governing the conformational preference. As a result of the net transfer of atomic population toward the exocyclic carbon and hydrogens (in 1p, relative to 1s), these atoms become importantly stabilized, a stabilization that outweighs the largely destabilized fluorine atom (Figure 3). Thus, the stabilization in the fluoromethyl group amounts to 3.8 kJ mol1, which surpasses the destabilization in the ring atoms, yielding to a slightly stabilized perpendicular structure. Benzylchloride. As for 1, two stationary points are found for this compound. The only minimum displays j = 90° (2p), whereas the restricted Cs optimization (j=0°) leads to a transition state with one imaginary frequency (2s) 8.3 kJ mol1 higher in energy (Figure 2). While C1C* and C*Cl bond lengths evolve according to the predictions of the stereoelectronic
model, this is not true for the C1C*Cl bond angle, which closes by 3.2° in 2p with regard to 2s, which is almost the reverse situation obtained for C2C1C*, clearly related with the release of Cl 3 3 3 H2 repulsions in 2p. ΔN(Ω) values are very similar to those observed in 1 (Table 2). There are three exceptions: H2, C*, and Z. ΔN(H2) is smaller for 2 than for 1 as the longer C*Cl bond length allows that Cl and H2 be at larger distances. This, combined with the more diffuse character of electron density in the chlorine basin (e.g., the averaged density is 0.106 au within a F basin and 0.098 au in Cl), diminishes the H2 3 3 3 Cl repulsion in 2s with regard to 1s. In this case, the CH2Cl group gains 0.026 au in 2p with regard to 2s, which is more than the corresponding amount obtained by CH2F on going from 1s to 1p (0.021 au). This can be explained considering that the repulsions with the π system are also smaller because of the same facts (geometry and most diffuse electron 13091
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The Journal of Physical Chemistry A density at Z). Therefore, Z can gain more electron density, as is also indicated by the fact that now the electron density is gained by the most electronegative atom, whereas in 1 the accumulation takes place at its attached carbon. Δδ(Ω,Ω0 ) values are also very similar in 2 and 1 (Table 3) if we exclude two cases: (i) Δδ(Z,C*), negligible for 1 and highly negative in 2, indicating that for the chlorinated derivative the repulsions with the π system in 2p do not preclude the halogen to recover the electron density lost because of the intense repulsions with H2 in 2s; and (ii) Δδ(Z,H2), much more negative in 2, basically because this index reaches a much higher value in 2s than in 1s, as a consequence of the larger volume of Cl, which gives rise to larger atomic overlap integrals for MOs. In this case, the stabilization of the p conformation involves smaller variations of atomic energy (Figure 4). Two atoms provide the largest stabilization, H2 and C*, whereas the halogen is again destabilized. Both the ring and CH2Z group are more stable in 2-p. Remaining Benzyl Derivatives (CH2X; X = OH, SH, NH2). The energy surface of molecules 35 is more complex than those of 1 and 2. Thus, we observe that diverse minima and gauche dispositions of C1C2C*Z (j) are obtained as the most stable conformer (3 and 4) or display relative energies below 1 kJ mol1 (5) (Figure 5). In all cases, conformations with p arrangement for j are obtained as true conformers, and one of them (5pt) is even the most stable in 5 (Table 1). Finally, s arrangements of j display diverse characteristics: being transition states for 3 and 4 (with a true conformer placed very close to it in 3) and minimum for 5. The fact that the most stable conformers of 3 and 4 adopt gauche dispositions for the C*Z bond with regard to the plane of the benzene ring can be highlighted as a first indication that the stereoelectronic model fails to explain conformational preferences in these compounds, as this disposition does not maximize the overlap between π MOs and σ*C*Z. We also highlight important differences in the energy profiles of compounds 1, 2, and 3 (Figure 2), which are dramatic in those conformations of 3 where χ is nearly antiperiplanar, 3-t conformations, because this arrangement allows us to establish a CO 3 3 3 HC intramolecular hydrogen bond in 3st conformations which compensates the
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smaller repulsions experienced in 3pt ones, providing a substantially flat energy profile (Figure 2). Also, as commented for 1 and 2, one of the geometry characteristics of the anomeric effect is not followed along the conformers of 35. In fact, C1C*Z closes from st to pt conformations (1.6° in 3, 4.1° in 4, and 3.2° in 5). Once more, the evolution of the C2C1C* bond angle in the series points to conformational preferences being more related to minimization of steric interactions (Table 1). ΔN(Ω) values computed for 3 (Table 5) follow trends very similar to those observed for 1 (Table 2). In fact, only one fact needs to be commented: ΔN(H2) and ΔN(Z) are larger in absolute value in 3. The explanation does not involve more favored delocalizations, it is purely steric: O 3 3 3 H2 interatomic distance is shorter (2.357 Å) in 3st than the corresponding F 3 3 3 H2 distance in 1s (2.383 Å). Moreover NBCCs of the OH group are closer to H2 in 3st than those of F in 1s. When comparing ΔN(Ω) values for 4 (Table 5) with those of another second row containing derivative, 2 (Table 2), we observe a larger set of significant differences than between first row elements (C1, C6, H2, C*, and Z). Nevertheless, all of them can be explained considering again Z 3 3 3 H2 and Z 3 3 3 π interactions. Despite that the S 3 3 3 H2 distance (2.590 Å) is shorter than the Cl 3 3 3 H2 one (2.637 Å), NBCCs around Cl in 2s are closer in average to H2 than those of S in 4st. As a consequence, the interactions between S and H2 do not expel a so large amount of electron density from both basins, and the alterations in Table 5. QTAIM ΔN(Ω) (in au and Multiplied by 103) and ΔE(Ω) (in kJ mol1) Values for pt Conformations Relative to st Ones in Molecules 35 ΔN(Ω) 3
4
5
3
4
5 4.8
C1
18
2
7
13.3
22.1
C2
0
1
2
3.5
3.0
0.1
C3
0
1
1
3.2
0.4
3.7
C4 C5
4 0
1 0
1 2
2.2 4.0
3.1 0.4
3.0 1.6
C6
12
5
1
3.1
3.4
1.5
H2
34
6
2
24.9
2.5
8.0
H3
2
0
2
2.2
0.4
2.4
H4
3
1
1
2.4
0.7
1.2
H5
3
1
1
2.7
1.3
1.1
H6
11
8
1
8.8
7.7
0.5
C* H*
10 9
12 6
7 8
32.7 9.4
1.1 6.6
25.6 8.1
20
7
17
65.7
8.4
36.9
11
5
4
12.9
0.7
2.3
Z
Figure 4. ΔE(Ω) values (in kJ mol1) for 2p with regard to 2s.
ΔE(Ω)
H(Z)
Figure 5. Most stable conformers for molecules 35. 13092
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Figure 6. Most stable conformers for molecules 68.
Table 6. Main B3LYP/6-311++G(2d,2p) 6d Optimized Geometry Parameters for Compounds 612a d(H 3 3 3 F)
j
E
Table 7. QTAIM ΔN(Ω) (in au and Multiplied by 103) Values for the Most Stable Conformers of Compounds 68 Relative to the Most Stable Planar Conformation (see Table 6) 6p6s
7g7s
8p8s
b
2.452
0.0
348.82592
6p
3.367
90.0
348.83437
C2
13
1
4
7sb
2.369
0.0
348.83738
7g
3.092
70.5
348.84068
C3 C4
6 3
12 1
21 14
7tb
2.712
0.0
348.83434
C5
5
16
12
8s
2.683
0.0
348.83710
C*
10
18
20
8g
3.455
100.0
348.83929
F
6
7
6
8tb 9sb
2.641 2.630c
180.0 0.0
348.83522 368.69110
16
2
H2
22
9g
3.086c
80.1
368.69895
H3
4
8
9t
2.786
180.0
368.69577
10s
2.743
0.0
368.69484
H4 H5
4 11
4 3
10 2
10g
3.336
110.6
368.69641
H*
1
14(s)
8(s)
10tb
2.656
180.0
368.69295
H*
1
1(t)
3(t)
11sb
2.071
0.0
524.59649
N1
2
0
0
11g 11t
2.573 2.324
68.2 180.0
524.60433 524.60359
12s
2.385
0.0
524.60563
12g
2.982
111.4
524.60617
12tb
2.371
180.0
524.60444
6s
a
Distances in Å and angles in degrees. Total molecular energies, E, in au. b Transition state. c Distances to the oxygen atom.
N(C1), N(C6), N(H2), and N(H6) are smaller. Finally, NBCCs in 4pt are oriented toward the benzene ring, which would result in larger electrostatic repulsions if ΔN(Z) reached a so large value as that in 2p. Molecule 5 displays the smallest ΔN(Ω) values for first row derivatives. This can be explained considering that 5st conformation places the nitrogen NBCC at the other side of H2, and H(Z) 3 3 3 H2 distances (2.415 Å) are significantly longer than O 3 3 3 H2 in 3st or F 3 3 3 H2 in 1s. Moreover, as previously found in diverse studies,24,25 the electron population at hydrogens bonded to nitrogen does not exceed 0.56 au, and their distributions are significantly polarized toward nitrogen. Fluoromethyl Derivatives of Other Aromatic Rings (Furan, Pyrrole, and Naphthalene). This study was complemented analyzing the conformational preferences of compounds that combine a CH2F group with other aromatic rings. We have considered the three position isomers of fluoromethylpyrrole (68). First, we notice that the only planar conformer corresponds to 3-fluoromethylpyrrole (8).The most stable minimum is always nonplanar (Figure 6), although it only could be named perpendicular for 1-fluoromethylpyrrole (6p), which is 22.2 kJ mol1
H1
15
more stable than the planar transition state 6s (Table 6), which is the largest energy variation due to this conformational change. According to ΔN(Ω) values (Table 7), the electron population of the CH2F group is only 0.002 au larger in 6p than in 6s. If the stabilization of the orthogonal conformation was due to a certain electron density transfer to this group, we should observe larger transfers for larger stabilizations. In contrast, once more we observe that the largest variations correspond to atoms involved in the main steric repulsion: H2, F, C2, and C*. The latter gains the electron density which cannot be accommodated in the F basin because of the increased repulsions with the π system. According to ΔE(Ω) values, the stabilization of 6p is provided by the ring, which becomes more stable by 65 kJ mol1, exceeding the destabilization of the CH2F group (43 kJ mol1). Only one atom, H2, displays an important variation in the Shannon entropy of the electron density distribution (0.057 au), accompanied by nonnegligible increases of the most external electron populations (those enclosed between the zero flux surface and the 0.002 au contour), which increase 0.005 au for H2 and 0.009 for F in 6p. This shows these atoms were involved in an important steric interaction in 6s. Similar trends pointing to mainly steric origin of conformational preferences are observed for 7 and 8, where energy differences between the most favored planar and nonplanar conformations are smaller (8.7 and 5.7, respectively). In this case, the molecular environment lacks of symmetry, and consequently two planar conformations are distinguished: 7s is preferred over 7t by 7.9 kJ mol1, although both are transition states. In contrast, 13093
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interactions (Figure 8); and (iii) the stabilization displayed by 11g with regard to 11s is much larger than that observed in 12 (Table 6), as the former implies releasing H8 3 3 3 F interaction (1.473 Å) and the latter that between H1 and F (2.318 Å).
Figure 7. Most stable conformers for molecules 9 and 10 indicating ΔN(Ω) variations with regard to the least energy planar conformation (9t and 10s). All values in au and multiplied by 103. Values in the range 0.005 to +0.005 au are not shown.
Figure 8. Most stable conformers for molecules 11 and 12 indicating main ΔN(Ω) variations with regard 11t and 12s, respectively. The rest of the naphthalene atoms experience variations below (0.003 au, accounting for a total variation of 0.004 au in 11g with regard to 11t and 0.008 au in 12g with regard to 12s.
and because of steric interactions between F and hydrogens attached to C2 and C4, 8t is more stable than 8s. In this case, 8t is another conformer and 8s is a transition state placed 5 kJ mol1 above 8t. Also, the different molecular environments at both sides of the plane containing the CC*F unit give rise to j angles that apart from 90°, yielding gauche arrangements for the main dihedral angle (Table 6). Two derivatives of furan, 2-fluoromethyl and 3-fluoromethyl furan (9 and 10, respectively), were also considered in this study. Both of them present two different planar stationary points. As a result of steric interactions between CH2F and neighboring hydrogens of furan, 9s and 10t are transition states, whereas 9t and 10s are true conformers. For both molecules, the most stable geometry corresponds to a nonplanar conformer displaying a nearly gauche CCC*F arrangement, named 9g and 10g, respectively. In both cases, the different molecular environments found at both sides of the plane containing the CC*F bond angle give rise (as already found in the pyrrole analogues 7 and 8) to gauche dispositions for j upon optimization. As shown in Figure 7, ΔN(Ω) values are clearly related to steric repulsions present in the conformations that are compared. ΔE(Ω) values indicate that in both furane derivatives the stabilization is provided by the CH2F unit. The balance between ΔE(C*) and ΔE(F) (5 kJ mol1) represents more than 50% of the relative stabilization of 9g. However, the stabilization of 10g would not take place without the role played by methylenic hydrogens (ΣΔE(H*) = 11 kJ mol1). In both cases, the energy of the furan ring remains practically unchanged. 1- and 2-fluoromethylnaphthalene (11 and 12) clearly rule out the stereoelectronic hypothesis because of several simple facts: (i) they do not adopt perpendicular dispositions of the C*F bond with regard to the naphthalene plane in the most stable conformers, whose arrangement is clearly gauche (11g and 12g); (ii) on the conformational change, significant ΔN(Ω) values are concentrated exclusively in the regions involving different steric
’ CONCLUSIONS The above discussion indicates that the conformational preference for perpendicular and gauche conformations along this series of aromatic derivatives is related to the diminution of the steric repulsion between the substituent and the closest hydrogen (or heteroatom in some cases) in the ring. The charge reorganization can be satisfactorily explained on the basis of the average distances between atomic nuclei of the phenyl ring and the NBCCs and nuclei of the substituent. The evolution of diverse atomic properties, as well as preferred geometries of some compounds and delocalization indices, indicates that the conformational preference is not mainly induced by electron delocalization. Moreover, electron delocalization is not significantly larger in the most stable conformations than in the completely planar ones. ’ ASSOCIATED CONTENT
bS
Supporting Information. QTAIM atomic electron populations for 1s and 2s and optimized bond lengths and bond angles related to anomeric effects for compounds 112. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ ACKNOWLEDGMENT We thank Centro de Supercomputacion de Galicia for the allocation of computational resources and Xunta de Galicia for funding this research through projects INCITE09E1R314091ES and INCITE08PXIB314224PR. A.V. also thanks Xunta de Galicia for one Licenza de estudos. ’ REFERENCES (1) Thacher, G. R. J. The Anomeric Effect and Associated Steroelectronic Effects; ACS Symposium Series No. 539; American Chemical Society: Washington DC, 1993. (2) Kirby, J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer Verlag: New York, 1983. (3) Juaristi, E.; Cuevas, G. The Anomeric Effect; CRC Press: Boca Raton, 1995. (4) Anderson, C. B.; Sepp, D. T. J. Org. Chem. 1967, 32, 607. (5) Penner, G. H.; Schaefer, R.; Sebastian, R.; Wolfe, S. Can. J. Chem. 1987, 65, 1845. (6) Utzat, K.; Restrepo, A. A.; Bohn, R. K.; Michels, H. H. Int. J. Quantum Chem. 2004, 100, 964. (7) Schaefer, T.; Schurko, R. W.; Sebastian, R.; Hruska, F. E. Can. J. Chem. 1995, 73, 816. (8) Sorenson, S. A.; True, N. S. J. Mol. Struct. 1991, 263, 21. (9) Celebre, G.; De Luca, G.; Longeri, M. Mol. Phys. 1989, 67, 239. (10) Bohn, R. K.; Sorenson, S. A.; True, N. S.; Brupbacher, T.; Gerry, M. C. L.; Jagger, W. J. Mol. Spectrosc. 1997, 148, 167. (11) Bader, R. F. W. Atoms in Molecules, A Quantum Theory; International Series of Monographs in Chemistry, No. 22, Oxford University Press: Oxford, 1990. (12) Bader, R. F. W. Chem. Rev. 1991, 91, 893. (13) Popelier, P. L. A. Atoms in Molecules, An Introduction; Prentice Hall: Essex, 2000. (14) Vila, A.; Mosquera, R. A. J. Comput. Chem. 2007, 28, 1516. 13094
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