Chemical Physics Letters 441 (2007) 194–197 www.elsevier.com/locate/cplett
On the change of the NMR isotropic shielding in blue- and red-shifted complexes of F3CH Sean A.C. McDowell
Department of Biological and Chemical Sciences, The University of the West Indies, Cave Hill Campus, Barbados Received 10 March 2007; in ﬁnal form 5 May 2007 Available online 10 May 2007
Abstract The change in the 1H and 13C isotropic magnetic shielding constant of F3CH on formation of the H-bonded F3CH Y complexes, Y = F, Cl, NH3, N(CD3)3, FH, ClH, OH2, SH2, N2, CO, OCO, Ar, NCH and O(CH3)2, were determined by GIAO MP2 ab initio and DFT computations. A good correlation between the sign of frequency shift and the change of the proton magnetic shielding was obtained, i.e., red-shifted complexes show the characteristic downﬁeld shift of the isotropic proton magnetic resonance while in blueshifted complexes the resonance is shifted upﬁeld or shifted downﬁeld but to a much smaller extent than in red-shifted complexes. Similar results were obtained for the 13C shifts. Counterpoise correction and basis set variation of the magnetic shielding were also considered. 2007 Elsevier B.V. All rights reserved.
1. Introduction Blue-shifting hydrogen bonding has been a subject of considerable interest in recent years since it has diﬀerent features to those normally associated with hydrogen bonds. A hydrogen bond may be represented symbolically by X–H Y where X is an element or fragment with a high electron aﬃnity and Y is a molecule containing a region of high electron density. In other words, X–H represents the proton donor and Y the proton acceptor. Normal features of hydrogen bonding include a diminution of the X–H stretch (a red shift), extension of the X–H bond and an increase in the infrared intensity of the X–H stretch . These features are usually reversed for blue-shifted hydrogen bonds, i.e., blue shift, X–H bond contraction and infrared intensity decrease are observed; see Ref.  for a review. In this Letter, signiﬁcant diﬀerences in 1H and 13C NMR isotropic shielding constants of blue-shifted complexes relative to red-shifted complexes are investigated since the features of the former species are quite diﬀerent from those of the latter. A downﬁeld shift of the proton magnetic res*
Fax: +1 246 417 4325. E-mail address: [email protected]
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.05.029
onance, ranging up to about 5 ppm for an X–H molecule going from the gas to liquid phase, is typical [3,4]. However, in a recent study of the ﬁve linear ClH Y dimers (Y = N2, CO, OC, BF, FB) it was noticed that the two blue-shifting species ClH OC and ClH FB have much smaller downﬁeld shifts on complexation . Therefore, it is natural to enquire whether this observation is more general and whether it holds for all blue-shifted complexes. We undertook a study of some complexes of triﬂuoromethane CF3H, which is a prototypical proton donor for blue-shifted hydrogen bonds but which also forms some red-shifted complexes. The proton acceptor in these complexes was varied to see what eﬀect this has on the magnetic shielding with respect to the uncomplexed CF3H molecule. We studied fourteen CF3H Y complexes, where Y = F, Cl, NH3, N(CD3)3, FH, ClH, OH2, SH2, N2, CO, OCO, Ar, NCH and O(CH3)2, with the ﬁrst four complexes being red-shifted and the remaining ten blue-shifted. Experimental data for the frequency shifts of some of these complexes are available for comparison with our computational results. For example, experimental FTIR studies of the spectra of CF3H dissolved in liquid Ar, N2, CO and CO2 show that the C–H stretching vibration of triﬂuoromethane is blue-shifted on complexation and is accompanied by a
S.A.C. McDowell / Chemical Physics Letters 441 (2007) 194–197
decrease in the corresponding v1 band [6,7]. Also, the C–H stretch of the complex of triﬂuoromethane and dimethyl ether in liquid argon appeared 17.7 cm1 above that of the monomer in the IR spectrum . The main aim of this study was to determine any correlation between the sign and magnitude of the computed frequency shift and the sign and magnitude of the change in the magnetic properties. We also looked speciﬁcally at the performance of the B3LYP functional of density functional theory (DFT) in the computation of the magnetic properties since this approach allows us to use larger basis sets which would be too computationally demanding for MP2 with the resources available to the author. Previous ab initio computations of the proton isotropic chemical shielding using the B3LYP functional with modest basis sets (like 6-31+G** and 6-311G**) for C–H O complexes  and in N-methyl maleimide-DMSO  were found to be in good agreement with MP2 and experiment. The proton shielding tensor in weakly bound X–H Y complexes can be approximated by Hð0Þ
H rH ab ¼ rab þ drab
where is the shielding tensor for the uncomplexed X–H molecule and drH ab is the change in the shielding tensor due to complexation. The isotropic shielding, deﬁned as r = (rxx + ryy + rzz)/3, is the observable of interest here. Eq. (1) allows us to evaluate changes in the isotropic shielding in red- and blue-shifted complexes by taking the diﬀerence between the ab initio values for the proton shielding tensors for X–H Y and X–H. The 13C shielding tensors were computed in similar fashion. The GIAO (gaugeinvariant atomic orbitals) method [11–13] was used to calculate the magnetic properties. The signiﬁcant contributions to drH ab come from the change in the tensor due to the non-uniform electric ﬁeld, including those due to high-frequency ﬂuctuations, arising HðelÞ from the proton acceptor Y rab , and from the magnetization induced in Y by the external magnetic ﬁeld HðmagÞ . Therefore, drH rab ab can also be approximated by , drH ab ¼
For the monoatomic (F, Cl, Ar) and the linear proton acceptors (like N2, CO and OCO), the magnetic contribuHðmagÞ tion rab can be estimated by computing the magnetizability, n, of Y. Assuming that the magnetization of Y can be represented as n times the magnetic ﬁeld then, in the long-range limit  that ignores contributions from higher induced magnetic multipole moments (like magnetic quadrupoles, etc.) , HðmagÞ
¼ ðl0 =4pÞT ac ncb
where l0 is the permeability of a vacuum (l0/ 4p = 107 JA2 m1) and Tac = (3RaRc R2dac)/R5. R is the distance from an origin in Y (either the nucleus of the monoatom or the centre of the linear molecule) to the H nu-
cleus. For the linear molecules, there are two independent components of the magnetizability, nk and n^, so the proton magnetic shielding is given by rHðmagÞ ¼ ðl0 =4pÞð2nk =R3 Þ zz 3 HðmagÞ HðmagÞ and rxx ¼ ryy = (l0/4p)(n^/R ) . From the comHðmagÞ HðelÞ , rab can be estimated puted values of drH ab and rab using Eq. (2). 2. Computational procedure All calculations were performed with the GAUSSIAN 03 suite of programs . The uncomplexed CF3H molecule and its complexes were ﬁrst optimized to stable structures at the MP2(full)/6-31+G(d,p) level of theory and the harmonic vibrational frequencies of the C–H stretch calculated, from which the frequency shift (Dx) was approximated as Dx = xcomplex xmonomer. The NMR computations were performed using the GIAO method with the same basis set. The computed changes in the C–H bond length, harmonic vibrational frequency and 1 H and 13C NMR isotropic shielding are shown in Table 1. The changes in molecular properties on complexation may be aﬀected by the basis set superposition error (BSSE), which is usually corrected by the Boys–Bernardi counterpoise correction . The BSSE is diminished by increasing the size of the basis set and should tend to zero in the limit of an inﬁnite basis set. We examined the basis set variation of the magnetic shielding in one of the complexes, CF3H FH, to assess the suitability of the 6-31+G(d,p) basis set for computing magnetic properties. The results for ﬁve basis sets of systematically increasing size are shown in Table 2. We also applied the counterpoise correction to the MP2 shielding with the 6-31+G(d,p) basis. Table 1 The change in the GIAO 1H and 13C isotropic NMR shielding constants ˚ ) and the C–H (drH, drC, in ppm), the C–H bond-length (drCH, in A harmonic vibrational stretch (dx, in cm1) for CF3H on formation of CF3H Y at the MP2(full)/6-31+G(d,p) level drH
CF3H F CF3H Cl CF3H NH3 CF3H N(CD3)3
5.982 2.142 0.387 0.022
10.325 3.887 1.988 2.118
CF3H FH CF3H ClH CF3H OH2 CF3H SH2 CF3H N2 CF3H CO CF3H OCO CF3H Ar CF3H NCH CF3H O(CH3)2
1.776 1.858 0.594 0.841 2.189 2.182 1.924 1.96 1.637 0.872
0.628 0.08 1.405 0.759 0.244 0.121 0.417 0.043 1.024 1.627
0.0482 0.0078 0.0009 0.0034
712.7 123.4 14.5 60.9
0.0009 0.0007 0.0016 0.0012 0.001 0.001 0.0015 0.0007 0.0016 0.0009
+18.4 +13.1 +33.1 +19.3 +19.1 +20.8 +30.3 +14.8 +32.6 +18.9
+14.1 +19.1 +23.6 +2.8 +17.7
Experimental frequency shifts (dxexpt) are included where available. For the uncomplexed CF3H molecule: rH = 23.3513 ppm, rC = 83.8808 ppm, ˚ , xe(C–H) = 3272.5 cm1. re(C–H) = 1.0839 A a Ref. . b Ref. . c Ref. .
S.A.C. McDowell / Chemical Physics Letters 441 (2007) 194–197
Table 2 Basis set variation of drH and drC for CF3H FH at the MP2(full) level of theory Basis set
6-31+G(d,p) 6-31++G(d,p) 6-311++G(d,p) 6-311++G(2d,2p) 6-311++G(2df,2pd)
1.776 0.226 2.749 0.241 0.247
0.628 0.652 0.696 0.814 0.800
Table 4 Comparison of the isotropic electric (rH(el)) and magnetic (rH(mag)) components of the isotropic proton shielding, and the change in the proton isotropic shielding (drH) for CF3H Y complexes with a linear C– H Y H-bond and with Y being either an atom or a linear molecule CF3H Y
B3LYP shielding values with this basis set and the much larger 6-311++G(2df,2pd) basis were also computed. These results are in Table 3. Finally, a comparison of the electric and magnetic components of the isotropic shielding for selected CF3H complexes are displayed in Table 4 and may shed light on the trends that are evident from the preceding tables.
CF3H F CF3H Cl CF3H Ar CF3H N2 CF3H CO CF3H OCO
8.433 5.166 0.126 0.049 0.119 0.179
0 0 0 0.302 0.31 0.155
drH 8.433 5.166 0.126 0.253 0.191 0.024
The following approximation was used, drH = rH(el) + rH(mag); drH is taken from the B3LYP/6-311++G(2df,2pd) computed values in Table 3, rH ðmagÞ ¼ ð2rHðmagÞ þ rHðmagÞ Þ=3 is obtained from computed magnetizxx zz abilities and rH(el) is the diﬀerence between drH and rH(mag). The B3LYP/ 6-311++G(2df,2pd) values for the magnetizabilities are: niso (F) = 2.7649 a.u.; niso (Cl) = 6.5187 a.u.; niso (Ar) = 4.3697 a.u.; nxx (N2) = 1.9851 a.u., nzz (N2) = 3.8385 a.u., niso (N2) = 2.6029 a.u.; nxx (CO) = 2.0598 a.u., nzz (CO) = 3.8376 a.u., niso (CO) = 2.6524 a.u., nxx (OCO) = 4.1988 a.u., nzz (OCO) = 5.5348 a.u., niso (OCO) = 4.6441 a.u.
The majority of the optimized CF3H Y complexes, Y = F, Cl, NH3, N(CD3)3, FH, ClH, OH2, SH2, N2, CO, OCO, Ar, NCH and O(CH3)2 have linear or nearly linear C–H Y bonds, i.e., with \C–H Y . 180 and Y represents the atom of the proton acceptor closest to the H atom of CF3H. The C–H Y bonds that are signiﬁcantly nonlinear are in CF3H N(CD3)3 (163), CF3H O(CH3)2 (158), CF3H ClH (118) and CF3H FH (113); in the last two complexes there are two H-bonded interactions, C–H F/Cl and F–H F, which are in the same plane and form a 5-membered ring structure. The intermolecular H Y separations range in ˚ (for CF3H F) to 3.0377 A ˚ magnitude from 1.5372 A (for CF3H ClH). Table 1 shows that the modest 6-31+G(d,p) basis set gives reasonably good agreement between experimental and theoretical frequency shifts, except for CF3H N(CD3)3
and CF3H Ar, where the computed shifts were overestimated. The expected correlation of C–H red shift/bond extension and blue shift/bond contraction is evident throughout, with the largest changes obtained for CF3H F and the smallest for CF3H ClH. However, a remarkable correlation between the change in the proton isotropic NMR shielding constant and the type of frequency shift is noticeable. The four red-shifted complexes show a downﬁeld shift in the proton magnetic resonances, while the blue-shifted complexes show an upﬁeld shift (except for CF3H O(CH3)2). The 13C resonances, on the other hand, all show a downﬁeld shift on complexation, but here also there is a distinction between the red- and blue-shifted species since the downﬁeld shift for the latter species is generally smaller in magnitude than for the former.
Table 3 Changes in the GIAO 1H and 13C isotropic NMR shielding constants (drH, drC, in ppm) for CF3H on formation of CF3H Y at the MP2(full)/631+G(d,p) level with counterpoise-correction (I), at the B3LYP/6-31+G(d,p) (II) and B3LYP/6-311++G(2df,2pd) (III) levels without counterpoise correction CF3H Y
CF3H F CF3H Cl CF3H NH3 CF3H N(CD3)3 CF3H FH CF3H ClH CF3H OH2 CF3H SH2 CF3H N2 CF3H CO CF3H OCO CF3H Ar CF3H NCH CF3H O(CH3)2
6.687 3.794 2.304 1.881 0.213 0.14 1.386 1.159 0.178 0.168 0.086 0.061 0.357 1.108
7.864 4.493 2.465 2.097 0.236 0.166 1.416 1.221 0.294 0.193 0.045 0.066 0.239 1.172
8.433 5.166 2.38 2.09 0.258 0.209 1.342 1.209 0.253 0.191 0.024 0.126 0.37 1.159
2.757 0.329 0.456 0.664 0.133 0.203 0.346 0.201 0.158 0.149 0.072 0.042 0.004 0.405
9.951 3.266 2.015 2.149 0.694 0.029 1.478 0.72 0.119 0.023 0.304 0.028 0.919 1.633
10.318 3.322 1.806 2.538 0.897 0.207 1.406 0.611 0.026 0.043 0.32 0.004 0.89 1.78
The MP2(full)/6-31+G(d,p) optimized geometries were used.
S.A.C. McDowell / Chemical Physics Letters 441 (2007) 194–197
The basis set variation of the magnetic shielding shown in Table 2 for CF3H FH prompted us to apply the counterpoise correction to the computation of dr, as well as using a larger basis set, so that a reasonable level of conﬁdence can be established for the observed trends in the magnetic properties. The results show that the 1H resonance is very sensitive to basis set while the 13C resonance is less so. For example, drH changes sign and drops by an order of magnitude in going from the 6-31+G(d,p) basis to the largest 6-311++G(2df,2pd) basis. By contrast, drC goes from 0.628 ppm to 0.800 ppm. It is interesting to note that a previous theoretical study of the gas-phase magnetic properties of ﬂuoromethanes, including CF3H, found that the proton shieldings were relatively insensitive to basis set size . Table 3 shows that counterpoise correction of the MP2/ 6-31+G(d,p) proton shielding gives good agreement with the uncorrected B3LYP/6-31+G(d,p) and B3LYP/ 6-311++G(2df,2pd) calculations; the latter calculation was considered to be relatively more accurate since a much larger basis set was used. The correlation between the change in vibrational frequency and magnetic shielding is still evident but now only CF3H N2 and CF3H CO show upﬁeld shifts while the other complexes show downﬁeld shifts. Nonetheless, there is still a clear distinction between the magnitude of drH and the sign of dx. The red-shifted complexes show a decrease in rH by more than about 1.8 ppm, whereas for all of the blue-shifted complexes, rH is never decreased by more than 1.4 ppm (and in most complexes by much less) or a small increase is predicted. Similar, though less pronounced, trends for the 13C resonances are also noticeable. Beyond the general trend elucidated above it is diﬃcult to ﬁnd an all-embracing reason for the variation in the magnitude of drH. For example, the C–H N interaction yields B3LYP/6-311++G(2df,2pd) drH values of 2.38 and 2.09 ppm, respectively, for the red-shifted CF3H NH3 and CF3H N(CD3)3 species, and 0.253 and 0.370 ppm, respectively, for the blue-shifted CF3H N2 and CF3H NCH species. Partitioning of the isotropic shielding into its electric and magnetic components for complexes with a linear C–H Y contact (Table 4) shows that in the blue-shifted complexes, the magnetic component is positive and either larger than or comparable in size to the negative electric component, thereby yielding a change in the shielding which is either positive or negative but small in magnitude. By contrast, for the red-shifted CF3H F and CF3H Cl species, the magnetic compo-
nent is isotropic (zero in magnitude) so that the large negative electric component (arising from the electric ﬁeld of the halide anion) dominates the shielding. In the blueshifted CF3H Ar, the magnetic component is also isotropic but the electric component is smaller since the electric ﬁeld arising from Ar is due to the polarization induced in the atom by the dipole moment of the CF3H molecule. Therefore, it yields a smaller negative magnetic shielding. The present work suggests that there is a signiﬁcant correlation between the type of frequency shift and the change in the proton (and 13C) isotropic magnetic shielding in Hbonded complexes. It is remarkable that the modest 631+G(d,p) basis set predicts striking diﬀerences between the red and blue-shifted complexes along the lines of downﬁeld and upﬁeld shifts, respectively. The diﬀerence manifests itself as a diﬀerence in relative magnitudes of the downﬁeld shifts with the use of a larger basis set. It would be interesting to extend this investigation to a wider range of H-bonded species. References  S. Scheiner, Hydrogen Bonding, Oxford University Press, New York, 1997.  P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253, and references contained therein.  G.C. Pimentel, A.L. McLellan, The Hydrogen Bond, Freeman, San Francisco, 1960.  J.A. Pople, W.G. Schneider, H.J. Bernstein, High-resolution Nuclear Magnetic Resonance, McGraw-Hill, New York, 1959.  S.A.C. McDowell, A.D. Buckingham, Mol. Phys. 104 (2006) 2527.  S.M. Melikova, K.S. Rutkowski, P. Rodziewicz, A. Koll, Chem. Phys. Lett. 352 (2002) 301.  S.M. Melikova, K.S. Rutkowski, P. Rodziewicz, A. Koll, J. Mol. Struct. 705 (2004) 49.  S.N. Delanoye, W.A. Herrebout, B.J. van der Veken, J. Am. Chem. Soc. 124 (2002) 7490.  S. Scheiner, Y. Gu, T. Kar, J. Mol. Struct. (THEOCHEM) 500 (2000) 441.  B. Wang, J.F. Hinton, P. Pulay, J. Phys. Chem. A 107 (2003) 4683.  F. London, J. Phys. Radium 8 (1937) 397.  R. Ditchﬁeld, Mol. Phys. 27 (1974) 789.  K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251.  A.D. Buckingham, Y. Tantirungrotechai, Mol. Phys. 96 (1999) 1217.  A.D. Buckingham, P.J. Stiles, Mol. Phys. 24 (1972) 99.  GAUSSIAN 03, Revision A.7, M.J. Frisch et al., Gaussian, Inc., Pittsburgh PA, 2003.  S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553. ˚ strand, K.V. Mikkelson, K. Ruud, T. Helgaker, J. Phys.  P.-O. A Chem. 100 (1996) 19771.  A. Kaldor, I.M. Mills, Spectrochim. Acta A 20 (1964) 523.