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A theoretical study of blue-shifting hydrogen bonds in π weakly bound complexes
Journal of Molecular Structure: THEOCHEM 908 (2009) 79–83
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Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
A theoretical study of blue-shifting hydrogen bonds in p weakly bound complexes B.G. Oliveira a,*, R.C.M.U. de Araújo a, M.N. Ramos b a b
Departamento de Química, Universidade Federal da Paraíba, 58059-900 João Pessoa, PB, Brazil Departamento de Química Fundamental, Universidade Federal de Pernambuco, 50739-901 Recife, PE, Brazil
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 16 April 2009 Accepted 7 May 2009 Available online 20 May 2009 Keywords: Blue-shifting Acetylene Ethylene Cyclopropene Tetrahedrene
a b s t r a c t In this work, a theoretical study about vibrational blue-shifting hydrogen bonds in p weakly bound complexes formed by acetylene (C2H2HCF3), ethylene (C2H4HCF3), cyclopropene (C3H4HCF3), tetrahedrene (C4H2HCF3), and fluoroform (HCF3) is presented. In these systems, the formation of the (pH) interaction occurs through the charge transfer from hydrocarbons to fluoroform via contact between their p bonds and hydrogen atoms, respectively. By taking into account calculations performed at the B3LYP/6311++G(d,p) level of theory, geometry results indicate a shortening of H–C bond of HCF3, where in infrared vibrational analysis, this structural observation is known as a blue-shifting stretch mode. In other words, it was observed that the H–C stretch frequency is shifted to upward wavenumbers accompanied by a reduction on the absorption intensity. Energetically, these p complexes are weakly bound because their intermolecular energies are very low, varying from 1.3 kJ mol 1 to 4.7 kJ mol 1. Moreover, a theoretical explanation for blue-shifts on H–C bonds of the fluoroform was presented through the evaluation of the ChelpG atomic charges, by which the quantification of charge transfer was used in order to justify the strengthening on (pH) hydrogen bond as follows: C2H2HCF3 > C2H4HCF3 > C3H4HCF3 > C4H2HCF3. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction It is well-established that non-covalent interactions play a dominant role in systems of chemical, physical and biological interest [1]. Over all these years, the hydrogen bonding has been considered as one of the most important non-covalent interactions [2]. By the advance in studies about specific interactions formed between a proton donor and a high electronic density center, nowadays a large number of intermolecular phenomena can be comprehended, such as those presents in conformation of bio-macromolecules [3], determination of transition states in organic reactions [4] and development of new pharmacologic drugs [5]. Physicochemically, the hydrogen bonding can be understood if analyze the postulates of electronic partition, which says that the total electronic energy and hence, the interaction energy or hydrogen bonding is ruled by electrostatic potential, polarizability effect, exchange spin terms, and charge transfer [2]. Notwithstanding, it is also well-known the relative contribution of these parameters to quantify the molecular energy [6], although in according with King and Weinhold [7], a theoretical study of molecular properties of HCN linear chains has shown the remarkable importance of the charge transfer. In hydrogen-bonded complexes, however, it is widely known that the concentration of charge density on monoprotic halogen acids yields drastic changes on their structures after complexation. One of these * Corresponding author. Tel.: +55 83 33219059. E-mail address: [email protected] (B.G. Oliveira). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.05.013
changes is the bond lengthening [8,9], by which its stretch frequency is shifted to a downward wave number followed of intense increases on absorption intensity [10,11]. Nevertheless, other kind of intermolecular interaction so-called blue-shifting hydrogen bonds have been studied by Hobza et al. [12]. In comparison with traditional hydrogen bonds, the blueshifting hydrogen bonds present an inverse behavior related to the proton donor, in which is observed a vibrational blue-shift instead of red-shift [13]. A long time ago, this phenomenon has been observed by Pinchas [14] and Sandorfy and co-workers [15]. By definition, it is verified on blue-shifting hydrogen bonds a strengthening of proton donor bonds because their stretch frequencies are dislocated to upward values [16–21]. Although it is very common to interpret blue-shifts in some specific proton donor centers, such as for instance the C–H bond of fluoroform (HCF3), nevertheless in this work we are admitting the possibility of the fluoroform to form intermolecular p complexes with acetylene (C2H2) and ethylene (C2H4), as well as cyclopropene (C3H4) and tetrahedrene (C4H2). In other words, we would like to say that besides the capacity of these unsaturated hydrocarbons to form typical hydrogen-bonded complexes with monoprotic halogen acids (HCl, HF and HCN), for instance [9,22], the characterization of blue-shifting hydrogen bonds [23] in p complexes such as C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3, in fact, still is needing of a careful investigation. Theoretically, it is known that hydrogen-bonded complexes and theirs molecular properties have been efficiently examined [24]
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through the calculations performed at the Density Functional Theory (DFT) [25] level of theory. Thereby, we expect that a hybrid functional [26–30] can describe the optimized geometries and electronic parameters, as well as the vibrational modes and blueshifts of the C–H bond of fluoroform molecule. Moreover, it is known that the quantification of electronic density is considered important, and sometimes, an essential parameter often used in studies of properties of intermolecular interactions, e.g., red-shifts effect [31]. In this insight, we will also include in this work the analysis of the charge transfer (DQ), whose nature is based from p bond of the hydrocarbons to fluoroform molecule upon the formation of the C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3 complexes. Aiming to obtain efficient results, the ChelpG algorithm [32,33] was chosen because it has yielded efficient results in recent studies about hydrogen-bonded complexes [34–36,11]. Well, ChelpG is based on calculation of electronic population by analysis of molecular electrostatic potential, what is a great advantage in comparison with other methods of charge density, where a lot of them were implemented either by non-trivial quantum-chemical methods or by arbitrary formulations [37]. 2. Computational methods The optimized geometries of the C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3 p complexes were obtained through the B3LYP/6-311++G(d,p) calculations executed on GAUSSIAN 98 W program [38]. The B3LYP method was chosen because this hybrid functional has been applied successfully in studies of intermolecular [39] and intramolacular systems [40], in special those characterized by blue-shifting hydrogen bonds [41]. The values of intermolecular energies were determined in according with supermolecule approach [2]. The intermolecular energy corrections, specifically the Basis Sets Superposition Error (BSSE) [42], it has been performed at light of the Counterpoise method developed by Boys and Bernardi. In complement, the ChelpG calculations were also executed on GAUSSIAN 98W program. 3. Results and discussion 3.1. Molecular parameters: structure, intermolecular energy and blueshift effects The optimized geometries of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p complexes are depicted in Fig. 1, whereas their main structural results are listed in Table 1. Initially, one important observation is the contraction of 0.0002 Å and 0.0001 Å for r(C–H) bonds of fluoroform, as well as the changes on their m(C–H) frequencies which are shifted upward in 5.8 cm 1, 3.2 cm 1, 1.8 cm 1 and 1.3 cm 1 for (a), (b), (c) and (d) complexes, respectively. In corroborating to that, it is also verified a significant diminution on the absorption intensities (I) of C–H bond, whose ratio values of I(C–H), c/I(C–H), m are in range to 0.12–0.63. Note that in C3H4HCF3 complex, the intensity of its C– H bond is drastically reduced from 32.8 km mol 1 to 4.0 km mol 1. As well-known, these vibrational alterations are blue-shifts [43,44], by which (a), (b), (c) and (d) are characterized as antihydrogen-bonded complexes or more usually named as blue-shifting hydrogen-bonded complexes. Based on works documented by specialized literature [45], the intermolecular energies of hydrogen complexes vary directly if are evidenced great alterations on the stretch frequencies of proton donors, essentially their red-shifts. Throughout this work, it has been debated that the blue-shifts on C–H bond of fluoroform are the main vibrational changes of the (a), (b), (c) and (d) p complexes. Thus, because (a) presents the largest Dm(C–H) value of
Fig. 1. Optimized geometries of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
Table 1 Structural parameters and infrared modes of the HCF3 monomer, as well as of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations. Parameters
Molecular systems HCF3
(a)
(b)
(c)
(d)
R(Hp) r(C—H) Dr(C—H) m(Hp) I(Hp) m(C–H) I(C–H) Dm(C–H) I(C–H), c/I(C–H), m
– 1.0896 – – – 3141.2 32.8 — —
2.774 1.0894 0.0002 53.4 0.2 3147 4.14 5.8 0.13
2.878 1.0894 0.0002 49.2 0.09 3144.4 4.0 3.2 0.12
2.950 1.0895 0.0001 44.0 0.014 3143.0 8.0 1.8 0.24
3.203 1.0895 0.0001 20.0 0.21 3142.5 20.5 1.3 0.63
Values of R and r are given in Å; values of m and I are given in cm respectively.
1
and km mol
1
,
5.8 cm 1, we would like to inform that a shortest R(Hp) distance of 2.774 Å has been computed for this complex. This lead us to admit that in comparison with ethylene, cyclopropene and tetrahedrene, the higher electronic density of the C„C bond of the acetylene provides the formation of a strongest p blue-shifting bound complex. In fact, the slight values of the m(Hp) new vibrational modes or commonly called as intermolecular stretch frequencies [10] are in corroboration with the intermolecular distance results presented above, e.g., in comparison with (b), (c) and (d) systems, the m(Hp) value of 53.4 cm 1 for (a) is the higher intermolecular stretch frequency. However, it is interesting to emphasize that the absorption intensity values of 0.2 km mol 1, 0.09 km mol 1, 0.014 km mol 1
B.G. Oliveira et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 79–83
and 0.21 km mol 1 for the (Hp) intermolecular modes of the correspondent (a), (b), (c) and (d) systems are very low, indicating that, even though these (Hp) blue-shifting hydrogen bonds are active vibrational modes, they are practically imperceptible in the infrared spectrum. Experimentally, either by low absorption intensities or slight Dm(C–H) results (1.3–5.8 cm 1), indeed the characterization of the (a), (b), (c) and (d) complexes is an encumbrance task, although it is fundamental to say that the blue-shift on C–H bond is the main criterion used to identify the formation of these intermolecular systems. However, the theoretical results debated here seem to be decisive to aid experimentalists in researches for p intermolecular systems, mainly if the blue-shift effects are point of investigation. In some works about hydrogenbonded complexes [46,47] is reported that intermolecular energy is a parameter often used to justify the intermolecular strength, or besides, if the charge transfer amount corroborates to that. By analyzing our results summarized in Table 2, firstly we would like to emphasize that moderate BSSE amounts corresponds to 10–25% of the uncorrected intermolecular energy DE. After correction, note that all DEC values are in range of 1.3–4.7 kJ mol 1, what lead us to consider these systems as weakly bound [48].
Table 2 Electronic parameters of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations. Parameters
Blue-shifting complexes (a)
DE BSSE DE C DQChelpG
5.20 0.50 4.70 0.036
(b)
(c)
5.12 0.62 4.50 0.035
Values of DE, BSSE and DEC are given in kJ mol electronic units (e.u.).
3.60 0.50 3.10 0.023 1
(d) 1.70 0.40 1.30 0.012
; values of DQChelpG are given in
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3.2. Charge transfer amount In a recent work, Zoub and co-workers [49] have shown that charge transfer is vital to predict equilibrium structures of hydrogen-bonded complexes. About the p weakly bound systems studied here, the quantities of charge transfer can aid us to interpret the results of the intermolecular energies DEC. In according with Fig. 2, we can see a satisfactory relationship between the corrected intermolecular energies (DEC) and their correspondent DQChelpG charge transfer amounts. However, in comparison with related works, which present electronic data and vibrational results of p hydrogen-bonded complexes [9], the quantification of charge transfer [50] serves as a guide to interpret the infrared spectra of proton donor species, in special their red-shifts. About this, Fig. 3 illustrates the relationship between the intermolecular stretch frequencies and charge transfer computed upon the formation of the (a), (b), (c) and (d) p blue-shifting complexes. In practice, the DQChelpG result of 0.063 e.u. shows that acetylene has a property to form stronger p complexes, of course due the high concentration of charge density on C„C bond, which is partially transferred to fluoroform. On the other hand, it is also admitted a charge distribution on C–H bond, although the DQChelpG amount is distributed on molecular surface of fluoroform as a whole, not solely concentrated on its C–H bond. Thus, the DQChelpG analysis indicates that (a) complex is the strongest bound system, even though the interaction strength for all p intermolecular systems studied here is ruled by following order: (a) > (b) > (c) > (d). Albeit it has been demonstrated that the m(Hp) intermolecular stretch frequencies are well correlated with the DQChelpG charge transfer, Fig. 4 illustrates that these vibrational modes also are well associated with the DEC corrected intermolecular energies. Well, we can perceive that among great DQChelpG charge transfer amounts, high DEC intermolecular energies, and finally stronger t(Hp) intermolecular stretch frequencies, in fact the analysis of these parameters must be effectuated by taking into account the quantification of the charge density on p center [51,52], where
Fig. 2. Relationship between the DEC intermolecular corrected energies and DQChelpG intermolecular charge transfer of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
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Fig. 3. Relationship between the infrared Dm(C–H) blue-shifting modes and DQChelpG intermolecular charge transfer of the C2H2HCF3 (a), C2H4HCF3 (b), C2H2 HCF3 (c) and C2H4HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
Fig. 4. Relationship between the DEC intermolecular corrected energies and their correspondent stretch frequencies m(Hp) of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
the C„C bond of acetylene provided the formation of a most stable complex (a), followed by C@C bonds of ethylene (b), and at last the cyclopropene (c) and tetrahedrene (d). 4. Conclusions In this work, a theoretical study about the molecular properties and infrared spectra of the C2H2HCF3, C2H4HCF3, C2H2 HCF3
and C2H4HCF3 p blue-shifting complexes was presented. From B3LYP/6-311++G(d,p) calculations, a structural evidence for blueshifting hydrogen bonds was demonstrated through the shortening of the C–H bonds of the fluoroform. In terms of spectroscopic parameters, it was observed deformations on C–H bonds, what leads to shifts on their stretch frequencies which are placed to upward wavenumbers or blue regions in infrared spectrum. About the electronic parameters, the values of the low intermolecular
B.G. Oliveira et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 79–83
energies are in satisfactory concordance with the m(Hp) weak intermolecular stretch frequencies. At last, the computation of larger blue-shifts for the C2H2HCF3 complex has been justified by stronger intermolecular energies DEC and higher charge transfers DQChelpG from p bond of the acetylene to the HCF3 molecule. Acknowledgements The authors gratefully acknowledge partial financial support from the CAPES and CNPq Brazilian funding agencies. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2009.05.013. References [1] S.J. Grabowski, Hydrogen Bonding–New insights, Springer, Berlin, 2006. [2] F.B. van Duijneveldt, J.N. Murrell, J. Chem. Phys. 46 (1967) 1759. [3] R. Flamia, G. Lanza, A.M. Salvi, J.E. Castle, A.M. Tamburro, Biomacromolecules 6 (2005) 1299. [4] H.D. Banks, J. Org. Chem. 68 (2003) 2639. [5] V.I. Teberekidis, M.P. Sigalas, J. Mol. Struct. (THEOCHEM) 803 (2007) 29. [6] H. Umeyama, K. Morokuma, J. Am. Chem. Soc. 99 (1977) 1316. [7] B.F. King, F. Weinhold, J. Chem. Phys. 103 (1995) 333. [8] R.C.M.U. Araújo, J.B.P. Silva, M.N. Ramos, Spectrochim. Acta A 51 (1995) b821. [9] R.C.M.U. Araújo, M.N. Ramos, J. Mol. Struct. (THEOCHEM) 366 (1996) 233. [10] D.J. Nesbitt, Chem. Rev. 88 (1988) 843. [11] B.G. Oliveira, M.L.A.A. Vasconcellos, R.R. Olinda, E.B.A. Filho, Struct. Chem. 20 (2009) 81. [12] P. Hobza, V. Špirko, Z. Havlas, K. Buckhold, B. Reiman, H.D. Barth, B. Brutschy, Chem. Phys. Lett. 299 (1999) 180. [13] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253. [14] S. Pinchas, Anal. Chem. 27 (1955) 2. [15] G. Trudeau, J.M. Dumas, P. Dupuis, M. Guérin, C. Sandorfy, Top. Curr. Chem. 93 (1980) 91. [16] B.G. Oliveira, E.C.S. Santos, E.M. Duarte, R.C.M.U. Araújo, M.N. Ramos, A.B. Carvalho, Spectrochim. Acta A 60 (2004) 1883. [17] B.G. Oliveira, E.M. Duarte, R.C.M.U. Araújo, M.N. Ramos, A.B. Carvalho, Spectrochim. Acta A 61 (2005) 491. [18] B.G. Oliveira, R.C.M.U. Araújo, A.B. Carvalho, M.N. Ramos, Spectrochim. Acta A 68 (2007) 626. [19] B.G. Oliveira, R.C.M.U. Araújo, M.N. Ramos, Struct. Chem. 19 (2008) 185. [20] B.G. Oliveira, R.C.M.U. Araújo, M.N. Ramos, Struct. Chem. 19 (2008) b665. [21] B.G. Oliveira, R.C.M.U. Araújo, F.C. Chagas, A.B. Carvalho, M.N. Ramos, J. Mol. Mod. 14 (2008) 949.
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A theoretical study of blue-shifting hydrogen bonds in p weakly bound complexes B.G. Oliveira a,*, R.C.M.U. de Araújo a, M.N. Ramos b a b
Departamento de Química, Universidade Federal da Paraíba, 58059-900 João Pessoa, PB, Brazil Departamento de Química Fundamental, Universidade Federal de Pernambuco, 50739-901 Recife, PE, Brazil
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 16 April 2009 Accepted 7 May 2009 Available online 20 May 2009 Keywords: Blue-shifting Acetylene Ethylene Cyclopropene Tetrahedrene
a b s t r a c t In this work, a theoretical study about vibrational blue-shifting hydrogen bonds in p weakly bound complexes formed by acetylene (C2H2HCF3), ethylene (C2H4HCF3), cyclopropene (C3H4HCF3), tetrahedrene (C4H2HCF3), and fluoroform (HCF3) is presented. In these systems, the formation of the (pH) interaction occurs through the charge transfer from hydrocarbons to fluoroform via contact between their p bonds and hydrogen atoms, respectively. By taking into account calculations performed at the B3LYP/6311++G(d,p) level of theory, geometry results indicate a shortening of H–C bond of HCF3, where in infrared vibrational analysis, this structural observation is known as a blue-shifting stretch mode. In other words, it was observed that the H–C stretch frequency is shifted to upward wavenumbers accompanied by a reduction on the absorption intensity. Energetically, these p complexes are weakly bound because their intermolecular energies are very low, varying from 1.3 kJ mol 1 to 4.7 kJ mol 1. Moreover, a theoretical explanation for blue-shifts on H–C bonds of the fluoroform was presented through the evaluation of the ChelpG atomic charges, by which the quantification of charge transfer was used in order to justify the strengthening on (pH) hydrogen bond as follows: C2H2HCF3 > C2H4HCF3 > C3H4HCF3 > C4H2HCF3. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction It is well-established that non-covalent interactions play a dominant role in systems of chemical, physical and biological interest [1]. Over all these years, the hydrogen bonding has been considered as one of the most important non-covalent interactions [2]. By the advance in studies about specific interactions formed between a proton donor and a high electronic density center, nowadays a large number of intermolecular phenomena can be comprehended, such as those presents in conformation of bio-macromolecules [3], determination of transition states in organic reactions [4] and development of new pharmacologic drugs [5]. Physicochemically, the hydrogen bonding can be understood if analyze the postulates of electronic partition, which says that the total electronic energy and hence, the interaction energy or hydrogen bonding is ruled by electrostatic potential, polarizability effect, exchange spin terms, and charge transfer [2]. Notwithstanding, it is also well-known the relative contribution of these parameters to quantify the molecular energy [6], although in according with King and Weinhold [7], a theoretical study of molecular properties of HCN linear chains has shown the remarkable importance of the charge transfer. In hydrogen-bonded complexes, however, it is widely known that the concentration of charge density on monoprotic halogen acids yields drastic changes on their structures after complexation. One of these * Corresponding author. Tel.: +55 83 33219059. E-mail address: [email protected] (B.G. Oliveira). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.05.013
changes is the bond lengthening [8,9], by which its stretch frequency is shifted to a downward wave number followed of intense increases on absorption intensity [10,11]. Nevertheless, other kind of intermolecular interaction so-called blue-shifting hydrogen bonds have been studied by Hobza et al. [12]. In comparison with traditional hydrogen bonds, the blueshifting hydrogen bonds present an inverse behavior related to the proton donor, in which is observed a vibrational blue-shift instead of red-shift [13]. A long time ago, this phenomenon has been observed by Pinchas [14] and Sandorfy and co-workers [15]. By definition, it is verified on blue-shifting hydrogen bonds a strengthening of proton donor bonds because their stretch frequencies are dislocated to upward values [16–21]. Although it is very common to interpret blue-shifts in some specific proton donor centers, such as for instance the C–H bond of fluoroform (HCF3), nevertheless in this work we are admitting the possibility of the fluoroform to form intermolecular p complexes with acetylene (C2H2) and ethylene (C2H4), as well as cyclopropene (C3H4) and tetrahedrene (C4H2). In other words, we would like to say that besides the capacity of these unsaturated hydrocarbons to form typical hydrogen-bonded complexes with monoprotic halogen acids (HCl, HF and HCN), for instance [9,22], the characterization of blue-shifting hydrogen bonds [23] in p complexes such as C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3, in fact, still is needing of a careful investigation. Theoretically, it is known that hydrogen-bonded complexes and theirs molecular properties have been efficiently examined [24]
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through the calculations performed at the Density Functional Theory (DFT) [25] level of theory. Thereby, we expect that a hybrid functional [26–30] can describe the optimized geometries and electronic parameters, as well as the vibrational modes and blueshifts of the C–H bond of fluoroform molecule. Moreover, it is known that the quantification of electronic density is considered important, and sometimes, an essential parameter often used in studies of properties of intermolecular interactions, e.g., red-shifts effect [31]. In this insight, we will also include in this work the analysis of the charge transfer (DQ), whose nature is based from p bond of the hydrocarbons to fluoroform molecule upon the formation of the C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3 complexes. Aiming to obtain efficient results, the ChelpG algorithm [32,33] was chosen because it has yielded efficient results in recent studies about hydrogen-bonded complexes [34–36,11]. Well, ChelpG is based on calculation of electronic population by analysis of molecular electrostatic potential, what is a great advantage in comparison with other methods of charge density, where a lot of them were implemented either by non-trivial quantum-chemical methods or by arbitrary formulations [37]. 2. Computational methods The optimized geometries of the C2H2HCF3, C2H4HCF3, C3H4HCF3 and C4H2HCF3 p complexes were obtained through the B3LYP/6-311++G(d,p) calculations executed on GAUSSIAN 98 W program [38]. The B3LYP method was chosen because this hybrid functional has been applied successfully in studies of intermolecular [39] and intramolacular systems [40], in special those characterized by blue-shifting hydrogen bonds [41]. The values of intermolecular energies were determined in according with supermolecule approach [2]. The intermolecular energy corrections, specifically the Basis Sets Superposition Error (BSSE) [42], it has been performed at light of the Counterpoise method developed by Boys and Bernardi. In complement, the ChelpG calculations were also executed on GAUSSIAN 98W program. 3. Results and discussion 3.1. Molecular parameters: structure, intermolecular energy and blueshift effects The optimized geometries of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p complexes are depicted in Fig. 1, whereas their main structural results are listed in Table 1. Initially, one important observation is the contraction of 0.0002 Å and 0.0001 Å for r(C–H) bonds of fluoroform, as well as the changes on their m(C–H) frequencies which are shifted upward in 5.8 cm 1, 3.2 cm 1, 1.8 cm 1 and 1.3 cm 1 for (a), (b), (c) and (d) complexes, respectively. In corroborating to that, it is also verified a significant diminution on the absorption intensities (I) of C–H bond, whose ratio values of I(C–H), c/I(C–H), m are in range to 0.12–0.63. Note that in C3H4HCF3 complex, the intensity of its C– H bond is drastically reduced from 32.8 km mol 1 to 4.0 km mol 1. As well-known, these vibrational alterations are blue-shifts [43,44], by which (a), (b), (c) and (d) are characterized as antihydrogen-bonded complexes or more usually named as blue-shifting hydrogen-bonded complexes. Based on works documented by specialized literature [45], the intermolecular energies of hydrogen complexes vary directly if are evidenced great alterations on the stretch frequencies of proton donors, essentially their red-shifts. Throughout this work, it has been debated that the blue-shifts on C–H bond of fluoroform are the main vibrational changes of the (a), (b), (c) and (d) p complexes. Thus, because (a) presents the largest Dm(C–H) value of
Fig. 1. Optimized geometries of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
Table 1 Structural parameters and infrared modes of the HCF3 monomer, as well as of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations. Parameters
Molecular systems HCF3
(a)
(b)
(c)
(d)
R(Hp) r(C—H) Dr(C—H) m(Hp) I(Hp) m(C–H) I(C–H) Dm(C–H) I(C–H), c/I(C–H), m
– 1.0896 – – – 3141.2 32.8 — —
2.774 1.0894 0.0002 53.4 0.2 3147 4.14 5.8 0.13
2.878 1.0894 0.0002 49.2 0.09 3144.4 4.0 3.2 0.12
2.950 1.0895 0.0001 44.0 0.014 3143.0 8.0 1.8 0.24
3.203 1.0895 0.0001 20.0 0.21 3142.5 20.5 1.3 0.63
Values of R and r are given in Å; values of m and I are given in cm respectively.
1
and km mol
1
,
5.8 cm 1, we would like to inform that a shortest R(Hp) distance of 2.774 Å has been computed for this complex. This lead us to admit that in comparison with ethylene, cyclopropene and tetrahedrene, the higher electronic density of the C„C bond of the acetylene provides the formation of a strongest p blue-shifting bound complex. In fact, the slight values of the m(Hp) new vibrational modes or commonly called as intermolecular stretch frequencies [10] are in corroboration with the intermolecular distance results presented above, e.g., in comparison with (b), (c) and (d) systems, the m(Hp) value of 53.4 cm 1 for (a) is the higher intermolecular stretch frequency. However, it is interesting to emphasize that the absorption intensity values of 0.2 km mol 1, 0.09 km mol 1, 0.014 km mol 1
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and 0.21 km mol 1 for the (Hp) intermolecular modes of the correspondent (a), (b), (c) and (d) systems are very low, indicating that, even though these (Hp) blue-shifting hydrogen bonds are active vibrational modes, they are practically imperceptible in the infrared spectrum. Experimentally, either by low absorption intensities or slight Dm(C–H) results (1.3–5.8 cm 1), indeed the characterization of the (a), (b), (c) and (d) complexes is an encumbrance task, although it is fundamental to say that the blue-shift on C–H bond is the main criterion used to identify the formation of these intermolecular systems. However, the theoretical results debated here seem to be decisive to aid experimentalists in researches for p intermolecular systems, mainly if the blue-shift effects are point of investigation. In some works about hydrogenbonded complexes [46,47] is reported that intermolecular energy is a parameter often used to justify the intermolecular strength, or besides, if the charge transfer amount corroborates to that. By analyzing our results summarized in Table 2, firstly we would like to emphasize that moderate BSSE amounts corresponds to 10–25% of the uncorrected intermolecular energy DE. After correction, note that all DEC values are in range of 1.3–4.7 kJ mol 1, what lead us to consider these systems as weakly bound [48].
Table 2 Electronic parameters of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations. Parameters
Blue-shifting complexes (a)
DE BSSE DE C DQChelpG
5.20 0.50 4.70 0.036
(b)
(c)
5.12 0.62 4.50 0.035
Values of DE, BSSE and DEC are given in kJ mol electronic units (e.u.).
3.60 0.50 3.10 0.023 1
(d) 1.70 0.40 1.30 0.012
; values of DQChelpG are given in
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3.2. Charge transfer amount In a recent work, Zoub and co-workers [49] have shown that charge transfer is vital to predict equilibrium structures of hydrogen-bonded complexes. About the p weakly bound systems studied here, the quantities of charge transfer can aid us to interpret the results of the intermolecular energies DEC. In according with Fig. 2, we can see a satisfactory relationship between the corrected intermolecular energies (DEC) and their correspondent DQChelpG charge transfer amounts. However, in comparison with related works, which present electronic data and vibrational results of p hydrogen-bonded complexes [9], the quantification of charge transfer [50] serves as a guide to interpret the infrared spectra of proton donor species, in special their red-shifts. About this, Fig. 3 illustrates the relationship between the intermolecular stretch frequencies and charge transfer computed upon the formation of the (a), (b), (c) and (d) p blue-shifting complexes. In practice, the DQChelpG result of 0.063 e.u. shows that acetylene has a property to form stronger p complexes, of course due the high concentration of charge density on C„C bond, which is partially transferred to fluoroform. On the other hand, it is also admitted a charge distribution on C–H bond, although the DQChelpG amount is distributed on molecular surface of fluoroform as a whole, not solely concentrated on its C–H bond. Thus, the DQChelpG analysis indicates that (a) complex is the strongest bound system, even though the interaction strength for all p intermolecular systems studied here is ruled by following order: (a) > (b) > (c) > (d). Albeit it has been demonstrated that the m(Hp) intermolecular stretch frequencies are well correlated with the DQChelpG charge transfer, Fig. 4 illustrates that these vibrational modes also are well associated with the DEC corrected intermolecular energies. Well, we can perceive that among great DQChelpG charge transfer amounts, high DEC intermolecular energies, and finally stronger t(Hp) intermolecular stretch frequencies, in fact the analysis of these parameters must be effectuated by taking into account the quantification of the charge density on p center [51,52], where
Fig. 2. Relationship between the DEC intermolecular corrected energies and DQChelpG intermolecular charge transfer of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
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Fig. 3. Relationship between the infrared Dm(C–H) blue-shifting modes and DQChelpG intermolecular charge transfer of the C2H2HCF3 (a), C2H4HCF3 (b), C2H2 HCF3 (c) and C2H4HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
Fig. 4. Relationship between the DEC intermolecular corrected energies and their correspondent stretch frequencies m(Hp) of the C2H2HCF3 (a), C2H4HCF3 (b), C3H4HCF3 (c) and C4H2HCF3 (d) p blue-shifting complexes using the B3LYP/6-311++G(d,p) calculations.
the C„C bond of acetylene provided the formation of a most stable complex (a), followed by C@C bonds of ethylene (b), and at last the cyclopropene (c) and tetrahedrene (d). 4. Conclusions In this work, a theoretical study about the molecular properties and infrared spectra of the C2H2HCF3, C2H4HCF3, C2H2 HCF3
and C2H4HCF3 p blue-shifting complexes was presented. From B3LYP/6-311++G(d,p) calculations, a structural evidence for blueshifting hydrogen bonds was demonstrated through the shortening of the C–H bonds of the fluoroform. In terms of spectroscopic parameters, it was observed deformations on C–H bonds, what leads to shifts on their stretch frequencies which are placed to upward wavenumbers or blue regions in infrared spectrum. About the electronic parameters, the values of the low intermolecular
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energies are in satisfactory concordance with the m(Hp) weak intermolecular stretch frequencies. At last, the computation of larger blue-shifts for the C2H2HCF3 complex has been justified by stronger intermolecular energies DEC and higher charge transfers DQChelpG from p bond of the acetylene to the HCF3 molecule. Acknowledgements The authors gratefully acknowledge partial financial support from the CAPES and CNPq Brazilian funding agencies. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2009.05.013. References [1] S.J. Grabowski, Hydrogen Bonding–New insights, Springer, Berlin, 2006. [2] F.B. van Duijneveldt, J.N. Murrell, J. Chem. Phys. 46 (1967) 1759. [3] R. Flamia, G. Lanza, A.M. Salvi, J.E. Castle, A.M. Tamburro, Biomacromolecules 6 (2005) 1299. [4] H.D. Banks, J. Org. Chem. 68 (2003) 2639. [5] V.I. Teberekidis, M.P. Sigalas, J. Mol. Struct. (THEOCHEM) 803 (2007) 29. [6] H. Umeyama, K. Morokuma, J. Am. Chem. Soc. 99 (1977) 1316. [7] B.F. King, F. Weinhold, J. Chem. Phys. 103 (1995) 333. [8] R.C.M.U. Araújo, J.B.P. Silva, M.N. Ramos, Spectrochim. Acta A 51 (1995) b821. [9] R.C.M.U. Araújo, M.N. Ramos, J. Mol. Struct. (THEOCHEM) 366 (1996) 233. [10] D.J. Nesbitt, Chem. Rev. 88 (1988) 843. [11] B.G. Oliveira, M.L.A.A. Vasconcellos, R.R. Olinda, E.B.A. Filho, Struct. Chem. 20 (2009) 81. [12] P. Hobza, V. Špirko, Z. Havlas, K. Buckhold, B. Reiman, H.D. Barth, B. Brutschy, Chem. Phys. Lett. 299 (1999) 180. [13] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253. [14] S. Pinchas, Anal. Chem. 27 (1955) 2. [15] G. Trudeau, J.M. Dumas, P. Dupuis, M. Guérin, C. Sandorfy, Top. Curr. Chem. 93 (1980) 91. [16] B.G. Oliveira, E.C.S. Santos, E.M. Duarte, R.C.M.U. Araújo, M.N. Ramos, A.B. Carvalho, Spectrochim. Acta A 60 (2004) 1883. [17] B.G. Oliveira, E.M. Duarte, R.C.M.U. Araújo, M.N. Ramos, A.B. Carvalho, Spectrochim. Acta A 61 (2005) 491. [18] B.G. Oliveira, R.C.M.U. Araújo, A.B. Carvalho, M.N. Ramos, Spectrochim. Acta A 68 (2007) 626. [19] B.G. Oliveira, R.C.M.U. Araújo, M.N. Ramos, Struct. Chem. 19 (2008) 185. [20] B.G. Oliveira, R.C.M.U. Araújo, M.N. Ramos, Struct. Chem. 19 (2008) b665. [21] B.G. Oliveira, R.C.M.U. Araújo, F.C. Chagas, A.B. Carvalho, M.N. Ramos, J. Mol. Mod. 14 (2008) 949.
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