| 1. |
- Majerz, Irena, et al.
(author)
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Asymmetric hydrogen bonds in a centrosymmetric environment. III. Quantum mechanical calculations of the potential-energy surfaces for the very short hydrogen bonds in potassium hydrogen dichloromaleate
- 2007
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In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63, s. 748-752
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Journal article (peer-reviewed)abstract
- In the crystal structure of potassium hydrogen dichloromaleate there are two short hydrogen bonds of 2.44 angstrom. The 'heavy-atom' structure is centrosymmetric ( space group P 3 (1) over bar) with centers of symmetry in the middle of the O-O bonds, suggesting centered hydrogen bonds. However, earlier unconventional types of refinements of the extensive neutron data taken at 30, 90, 135, 170 and 295 K demonstrated that the H atoms are actually non-centered in the hydrogen bonds, although the environment is centrosymmetric. Traditionally it has been assumed that the hydrogen distribution adopts the same symmetry as the environment. Reviewing these unusual results it was considered of great interest to verify that the non-centered locations of the H atoms are reasonable from an energy point of view. Quantum mechanical calculations have now been carried out for the potential-energy surfaces ( PES) for both the centered and non-centered locations of the H atoms. In all cases the non-centered positions are closer to the energy minima in the PES than the centered positions, and this result confirms that the structure is best described with noncentered H atoms. There is virtually perfect agreement between the quantum-mechanically derived reaction coordinates ( QMRC) and the bond-order reaction coordinates ( BORC) derived using Pauling's bond-order concept together with the principle of conservation of bond order.
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| 2. |
- Majerz, Irena, et al.
(author)
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Comparison of the proton-transfer path in hydrogen bonds from theoretical potential-energy surfaces and the concept of conservation of bond order. II. (N—H…N)+ hydrogen bonds
- 2007
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In: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 63:4, s. 650-662
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Journal article (peer-reviewed)abstract
- The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in (N—H—N)+ hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between the QMRC and the corresponding bond-order reaction coordinates (BORC) derived by applying the Pauling bond-order concept together with the principle of conservation of bond order. We find virtually perfect agreement between the QMRC and the BORC for intermolecular (N—H—N)+ hydrogen bonds. In contrast, for intramolecular (N—H—N)+ hydrogen bonds, the donor and acceptor parts of the molecule impose strong constraints on the N—N distance and the QMRC does not follow the BORC relation in the whole range. The X-ray determined hydrogen positions are not located exactly at the theoretically calculated potential-energy minima, but instead at the point where the QMRC and the BORC coincide with each other. On the other hand, the optimized hydrogen positions, with other atoms in the cation fixed as in the crystal structure, are closer to these energy minima. Inclusion of the closest neighbours in the theoretical calculations has a rather small effect on the optimized hydrogen positions. [Part I: Olovsson (2006). Z. Phys. Chem.220, 797–810.]
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| 3. |
- Majerz, Irena, et al.
(author)
-
Comparison of the proton-transfer paths in hydrogen bonds from theoretical potential-energy surfaces and the concept of conservation of bond order III. O-H-O hydrogen bonds
- 2010
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 12:20, s. 5462-5467
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Journal article (peer-reviewed)abstract
- The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in O-H-O hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between the QMRC and the corresponding bond-order reaction coordinates (BORC) derived by applying the Pauling bond order concept together with the principle of conservation of bond order. In agreement with earlier results for N-H-N+ hydrogen bonds there is virtually perfect agreement between the QMRC and BORC curves for intermolecular O-H-O hydrogen bonds. For intramolecular O-H-O hydrogen bonds, the donor and acceptor parts of the molecule impose strong constraints on the O center dot center dot center dot O distance and the QMRC does not follow the BORC relation in the whole range. The neutron-determined proton positions are located close to the theoretically calculated potential-energy minima, and where the QMRC and the BORC curves coincide with each other. The results confirm the universal character of intermolecular hydrogen bonds: BORC is identical with QMRC and the proton can be moved from donor to acceptor keeping its valency equal to 1. The shape of PES for intramolecular hydrogen bonds is more complex as it is sensitive to the geometry of the molecule as well as of the hydrogen bridge.
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| 4. |
- Majerz, Irena, et al.
(author)
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Influence of Proton Transfer Degree on the Potential Energy Surface for Two Very Short Hydrogen Bonds
- 2011
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In: Acta Chimica Slovenica. - 1318-0207 .- 1580-3155. ; 58:3, s. 379-384
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Journal article (peer-reviewed)abstract
- The influence of temperature on the proton location in hydrogen bonds has been systematically studied by neutron diffraction in only a few crystal structures. Two of these are the 1: 1 complex of urea - phosphoric acid with an OHO hydrogen bond and 4-methylpyridine-pentachlorophenol with an OHN hydrogen bond. Based on these earlier determined crystal structures the potential energy surface (PES) at different temperatures has now been determined by DFT calculations at the B3LYP/6-31++G** level of theory using the Gaussian03 system. In general PES is practically unchanged as the proton moves from the donor to the acceptor. This is not surprising as the crystal structure does not undergo significant changes as the proton successively moves along the hydrogen bond. For both complexes PES is characterized by only one minimum, which is not located at the centre where the distances of the proton to the bridge atoms are the same. The experimental proton positions are located close to the calculated energy minima; the slight deviations are probably an effect of the crystalline environment which has not been taken into account in the calculations.
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| 5. |
- Majerz, Irena, et al.
(author)
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Influence of proton transfer on the geometry of the donor and acceptor in NHN+ hydrogen bonds
- 2010
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In: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 976:1-3, s. 11-18
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Journal article (peer-reviewed)abstract
- The correlation between distances X-H and H-Y in hydrogen bonds has been studied extensively but it is difficult to determine experimentally individual changes in the internal structure of the molecules involved in the hydrogen bonds. In the present study the influence of proton transfer on the geometry of donors and acceptors in NHN+ hydrogen bonds has been investigated by theoretical calculations at the B3LYP/6-31++G** level of theory using the Gaussian03 system. The selected compounds provide examples of both inter- and intramolecular hydrogen bonds. Typically, the largest changes are in the bonds involving the closest neighbors to the NHN bridge atoms, but in certain cases the geometrical features of parts that are more remote from the bridge atoms are also affected, and can result in significant changes of the conformation of the molecules.
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| 6. |
- Majerz, Irena, et al.
(author)
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Proton transfer in the intramolecular NHN+ bonds in proton sponges with different hydrogen bridge flexibility
- 2009
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 11:9, s. 1297-1302
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Journal article (peer-reviewed)abstract
- The proton transfer in the intramolecular NHN+ hydrogen bonds of selected proton sponges has been studied using theoretical calculations of the potential energy surfaces (PES). The proton-transfer trajectory follows very closely the lowest energy path, derived as the quantum-mechanical reaction coordinates (QMRC). The bond order is not conserved in the transfer process. Even in the most flexible proton sponges there are considerable constraints on the NN distance and the hydrogen bonds do not behave as intermolecular bonds. The curvature of QMRC is not a suitable criterion to distinguish between inter- and intramolecular NHN+ bonds. It appears that the determining factor for linearity is the degree of constraint, which is most likely the strongest in the benzene proton sponges. In the naphthalene proton sponges with relatively short NN distances QMRC is more bent than in the benzene complexes with somewhat longer distances, opposite to what might be expected. It is important to note that in intramolecular complexes the PES is characterized by a single minimum, in contrast to a double minimum in intermolecular complexes. The experimentally determined NH bond lengths have been plotted on the potential energy surface and these points are all located on the QMRC curve, very close to the energy minimum of the PES. However, it is vital that the experimental X-ray hydrogen positions are then corrected to give the true internuclear NH distances.
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| 7. |
- Majerz, Irena, et al.
(author)
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Proton-transfer paths in CH···O hydrogen bonds
- 2012
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In: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 2:6, s. 2545-2552
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Journal article (peer-reviewed)abstract
- Potential energy surfaces for a series of intermolecular CH center dot center dot center dot O hydrogen bonds have been calculated in order to determine the Quantum Mechanical Reaction Coordinates (QMRCs). The results have shown that one QMRC curve is common for strong C-H center dot center dot center dot O hydrogen bonds, and another for very weak interactions. For intermediate hydrogen bonds the shape of the potential energy curve depends on the particular type of the C-H center dot center dot center dot O bond, which is related to the proton donor ability and geometry of the hydrogen bridge.
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| 8. |
- Majerz, Irena, et al.
(author)
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The influence of the OHO angle on the proton valency in inter- and intra-molecular hydrogen bonds
- 2010
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In: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 968:1-3, s. 48-51
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Journal article (peer-reviewed)abstract
- The influence of the OHO angle in the hydrogen bond on the potential-energy surface has been studied. The OHO angle has been artificially changed from 140 degrees to 180 degrees to study the influence of the angle on the location of the energy minimum relative to QMRC, the minimum energy reaction path, and BORC, the bond-order reaction path. In the intermolecular hydrogen bond QMRC and BORC are identical, independent on the OHO angle. The intramolecular hydrogen bond is very sensitive to a change in the OHO angle. The agreement between the theoretical results is confirmed by available experimental neutron data.
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| 9. |
- Majerz, Irena, et al.
(author)
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The shape of the potential energy curves for NHN+ hydrogen bonds and the influence of non-linearity
- 2008
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 10:21, s. 3043-3051
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Journal article (peer-reviewed)abstract
- The potential energy curves for proton motion in NHN+ hydrogen bonds have been calculated to investigate whether different methods of evaluation give different results: for linear H bonds most curves calculated along the NH direction are, as expected, identical with those along NN; for intramolecular H bonds it is very important to take into account the non-linearity and the potential energy curve calculated along the NH direction can be very far from the curve correctly describing the proton transfer. Other factors which influence the proton-transfer process are steric hindrance and presence of anions which modify the proton motion. In the analysis of the proton transfer process it is very important to take changes in the structure of the rest of the molecule into account, which is connected with exchange of energy with the surroundings. Comparison of adiabatic and non-adiabatic curves shows that they are significantly different for very bent hydrogen bonds and for hydrogen bonds with steric constraints for which the proton transfer process must be accompanied with relaxation of the whole molecule. Comparison of the potential-energy curves for compounds with very short H bonds emphasizes that the term strong H bond needs to be qualified. For intermolecular H bonds shortening of the bond is connected with linearization. But for intramolecular H bonds the NN distance cannot be used as the only measure of H bond strength.
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