Calculations quantum mechanical

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. [Pg.63]

For the trimethylspironaphthoxazine, ab initio molecular orbital (MO) calculations indicated that the most stable colored form is the trans-trans-cis- form - about 7 kcal-mol 1 endothermic relative to the spiro form. Measurement of the proton NMR nuclear Overhauser effect experimentally confirmed this calculated result. The structural calculations indicate that the colored form is essentially quinoidal, rather that zwitterionic.186 [Pg.63]

The electronic spectra of spiropyrans of many kinds and those of the related 277-chromenes have been reviewed. 7 The relative energy levels of the ground and [Pg.63]

In variational calculations in quantum mechanics, one is interested in minimizing the energy expectation [Pg.182]

For certain choices of trial wave functions further simplifications are possible, and this has been exploited in an interesting way in a series of variational calculations on the ground states of He and He, begun by McMillan and continued by the Orsay group. The model is again that of N particles with pair interactions, confined to a box, and with periodic boundary conditions to mimic an infinite system. The wave function has to be able to prevent strong overlaps of the particles, and a popular form for the boson case is therefore simply a product of pair functions. [Pg.183]

One of the main aims of quantum mechanical methods in chemistry is the calculation of energies of molecules as a function of their geometries. This requires the generation of potential energy hypersurfaces. If these surfaces can be calculated with sufficient accuracy, they may be employed to predict equilibrium geometries of molecules, relative energies of isomers, the rates of their interconversions, NMR chemical shifts, vibrational spectra, and other properties. Carbocations are ideally suited for calculations since relative energies of well-defined structural isomers are frequently not easily determined experimentally. It should be kept in mind, however, that theoretical calculations usually refer to isolated ion structures in the idealized gas phase. [Pg.195]

Over the years, several computational methods have been developed. The variational theory can be used either without using experimental data other than the fundamental constants (i.e., ab initio methods) or by using empirical data to reduce the needed amount of numerical work (i.e., semiempirical data methods). There are various levels of sophistication in both ab initio [HF(IGLO), density functional theory (DFT) GIAO-MP2, GIAO-CCSD(T)] and semiempirical methods. In the ab initio methods, various kinds of basic sets can be employed, while in the semiempirical methods, different choices of empirical parameters and parametric functions exist. Tire reader is referred to reviews of the subject. [Pg.195]

Recent developments in computational chemistry allow to establish the exact structure of carbocations by combining computational and experimental results. Furthermore, accurate A NMR chemical shifts of carbocations and other organic molecules can be calculated with the application of recent coupled cluster methods, for example, GIAO-CCSD(T). [Pg.195]

Alkonium Ions Incorporating Bridging Hydrogens (Protonated [Pg.195]

The pioneering work of Meerwein, Ingold, and Whitmore demonstrated that trivalent alkyl cations (C H2 +T) play an important role in the acid-catalyzed [Pg.195]

The calculation of the properties of a solid via quantum mechanics essentially involves solving the Schrodinger equation for the collection of atoms that makes up the material. The Schrodinger equation operates upon electron wave functions, and so in quantum mechanical theories it is the electron that is the subject of the calculations. Unfortunately, it is not possible to solve this equation exactly for real solids, and various approximations have to be employed. Moreover, the calculations are very demanding, and so quantum evaluations in the past have been restricted to systems with rather few atoms, so as to limit the extent of the approximations made and the computation time. As computers increase in capacity, these limitations are becoming superseded. [Pg.76]

The use of quantum mechanical calculations of solid properties was initially the province of solid-state physics, and the calculation of electron energy levels in metals and semiconductors is well established. Chemical quantum mechanical [Pg.76]

The previous sections of this chapter have established that NEMCA, or Electrochemical Promotion, is caused by the electrochemically controlled backspillover of ionic species onto the catalyst surface and by the concomitant change on catalyst work function and adsorption binding energies. Although the latter may be considered as a consequence of the former, experiment has shown some surprisingly simple relationships between change AO in catalyst [Pg.267]

AEads aadsAO aads 0 for electron acceptor adsorbates aads 0 for electron donor adsorbates (5.72) [Pg.267]

AEact aactAO OLac O for electrophobic reactions aact 0 for electrophilic reactions (5.73) [Pg.267]

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function D are equal to each other and equal to the increase in O. Similar is the case for ethylene epoxidation and deep oxidation on Ag.5 [Pg.268]

Although the range of validity of such simple expressions may not be too wide (typically 0.3-leV) still it is quite important to try to analyze and understand them as rigorously as possible. [Pg.268]

An essential rule is that descriptors should be calculated by the same level of theory for all molecules in a given data set. Trends in computed quantities have a chance to be consistent within a given computational method but not across different methods. For example, AMI and MNDO atomic charge values may differ but within the framework of either method the relative order of numerical values may be similar. [Pg.220]

Note that the QM molecular parameters generally apply to the electronic ground state for a single conformation of the isolated system at 0 K. Properties [Pg.220]

Some of the more common descriptors with their common symbols are listed in Tables 1-3. The section on Examples of LEER Equations illustrates and explains some of these. Because of the need for consistency in this chapter, the symbols used here might not always match those used in the original articles. Nonetheless, we have endeavored to retain their meaning as faithfully as possible. Descriptors, along with their symbols, often tend to evolve as their application changes. [Pg.221]

Symbols Meaning or Definition Occurs in Equation Number [Pg.221]

Symbol Name or Description Occurs in Equation Numbers [Pg.222]

In summary, the potential models proposed so far for MD simulations of zeolites were able to reproduce quite well structural properties of the frameworks in general, but did not always reach a refiable prediction of the vibrational spectra. The dynamic behavior is obviously much more sensitive to the chosen potential function than structural characteristics. From our point of view, this leads to the conclusion that the accurate reproduction of experimental vibrational spectra should be taken as one of the key criteria in further force field developments. [Pg.24]

The methods discussed so far are based on the principles of classical mechanics and necessitate the use of experimental data in the parametrization of the potential model chosen. An alternative approach implies that results of quantum mechanical (QM) calculations are employed as such experimental data. Within limits this can allow us to avoid the use of any experimental information other than the values of fundamental physical constants in quantum mechanical ab initio calculations. Alternatively, information about vibrational spectra are also accessible on the direct way from QM calculations. [Pg.24]

Any problem concerning the electronic structure of matter is governed by the well-known Schrodinger equation, and for systems without time-dependent interactions the time-independent Schrodinger equation given by [Pg.24]

The connecting link between ab initio calculations and vibrational spectra is the concept of the energy surface. In harmonic approximation, usually adopted for large systems, the second derivatives of the energy with respect to the nuclear positions at the equilibrium geometry give the harmonic force constants. For many QM methods such as Hartree-Fock theory (HF), density functional methods (DFT) or second-order Moller-Plesset pertiubation theory (MP2), analytical formulas for the computation of the second derivatives are available. However, a common practice is to compute the force constants numerically as finite differences of the analytically obtained gradients for small atomic displacements. Due to recent advances in both software and computer hardware, the theoretical determination of force field parameters by ab initio methods has become one of the most common and successful applications of quantum chemistry. Nowadays, analysis of vibrational spectra of wide classes of molecules by means of ab initio methods is a routine method [85]. [Pg.25]

Although the harmonic approximation is satisfactory for small displacements from the equifibrium position, ab initio harmonic force constants and vibrational frequencies are known to be typically overestimated as compared with those experimentally found [86]. Sources of this disagreement are the omission or incomplete incorporation of electron correlation, basis set deficiencies, and the neglect of anharmonicity effects. However, as the overestimation is fairly uniform, the appHcation of appropriate scahng procedures becomes feasible. Due to its simplicity, global scafing (using one uniform scale factor determined by a least-squares fit of the calculated to the experimental vibrational frequencies) has widely been used at different levels of theory [87]. However, for most spectro- [Pg.25]

Tannor D J, Kosloff R and Rice S A 1986 Coherent pulse sequence induced control of selectivity of reactions exact quantum mechanical calculations J. Chem. Rhys. 85 5805-20, equations (1)-(6)... [Pg.279]

Wliether the potentials are derived from quantum mechanical calculations or classical image forces, it is quite generally found that there is a stronger barrier to the adsorption of cations at the surface than anions, in agreement with that generally. ... [Pg.596]

Manthe U, Seideman T and Miller WH 1993 Full-dimensional quantum-mechanical calculation of the rate-constant for the H2 + OH H2O + H reaction J. Chem. Phys. 99 10 078-81... [Pg.1004]

Olsen R A, Philipsen P H T, Baerends E J, Kroes G J and Louvik O M 1997 Direct subsurface adsorption of hydrogen on Pd(111) quantum mechanical calculations on a new two-dimensional potential energy surfaced. Chem. Phys. 106 9286... [Pg.2236]

In Figure 1, we see that there are relative shifts of the peak of the rotational distribution toward the left from f = 12 to / = 8 in the presence of the geometiic phase. Thus, for the D + Ha (v = 1, DH (v, f) - - H reaction with the same total energy 1.8 eV, we find qualitatively the same effect as found quantum mechanically. Kuppermann and Wu [46] showed that the peak of the rotational state distribution moves toward the left in the presence of a geometric phase for the process D + H2 (v = 1, J = 1) DH (v = 1,/)- -H. It is important to note the effect of the position of the conical intersection (0o) on the rotational distribution for the D + H2 reaction. Although the absolute position of the peak (from / = 10 to / = 8) obtained from the quantum mechanical calculation is different from our results, it is worthwhile to see that the peak... [Pg.57]

The relative shift of the peak position of the rotational distiibution in the presence of a vector potential thus confirms the effect of the geometric phase for the D + H2 system displaying conical intersections. The most important aspect of our calculation is that we can also see this effect by using classical mechanics and, with respect to the quantum mechanical calculation, the computer time is almost negligible in our calculation. This observation is important for heavier systems, where the quantum calculations ai e even more troublesome and where the use of classical mechanics is also more justified. [Pg.58]

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... [Pg.365]

The principal idea behind the CSP approach is to use input from Classical Molecular Dynamics simulations, carried out for the process of interest as a first preliminary step, in order to simplify a quantum mechanical calculation, implemented in a subsequent, second step. This takes advantage of the fact that classical dynamics offers a reasonable description of many properties of molecular systems, in particular of average quantities. More specifically, the method uses classical MD simulations in order to determine effective... [Pg.367]

It was reahzed quite some decades ago that the amount of information accumulated by chemists can, in the long run, be made accessible to the scientific community only in electronic form in other words, it has to be stored in databases. This new field, which deals with the storage, the manipulation, and the processing of chemical information, was emerging without a proper name. In most cases, the scientists active in the field said they were working in "Chemical Information . However, as this term did not make a distinction between librarianship and the development of computer methods, some scientists said they were working in "Computer Chemistry to stress the importance they attributed to the use of the computer for processing chemical information. However, the latter term could easily be confused with Computational Chemistry, which is perceived by others to be more limited to theoretical quantum mechanical calculations. [Pg.4]

Clearly then, the understanding of chemical reactions under such a variety of conditions is still in its infancy and the prediction of the course and products of a chemical reaction poses large problems. The ab initio quantum mechanical calculation of the pathway and outcome of a single chemical reaction can only be... [Pg.169]

Inadequate availability of experimental data can considerably inhibit the development of improved energy functions for more accurate simulations of energetic, structural, and spectroscopic properties. This has led to the development of class II force fields such as CFF and the Merck Molecular Force Field (MMFF), which are both based primarily on quantum mechanical calculations of the energy surface. The purpose of MMFF, which has been developed by Thomas Halgren at Merck and Co., is to be able to handle all functional groups of interest in pharmaceutical design. [Pg.355]

The GB equation is suitable for the description of solvent effects in molecular mechanics and dynamics [16], as well as in quantum mechanical calculations (17,18]. An excellent review of implicit solvation models, with more than 900 references, is given by Cramer and Truhlar [19]. [Pg.365]

The MEP at the molecular surface has been used for many QSAR and QSPR applications. Quantum mechanically calculated MEPs are more detailed and accurate at the important areas of the surface than those derived from net atomic charges and are therefore usually preferable [Ij. However, any of the techniques based on MEPs calculated from net atomic charges can be used for full quantum mechanical calculations, and vice versa. The best-known descriptors based on the statistics of the MEP at the molecular surface are those introduced by Murray and Politzer [44]. These were originally formulated for DFT calculations using an isodensity surface. They have also been used very extensively with semi-empirical MO techniques and solvent-accessible surfaces [1, 2]. The charged polar surface area (CPSA) descriptors proposed by Stanton and Jurs [45] are also based on charges derived from semi-empirical MO calculations. [Pg.393]

This is a question of reaction prediction. In fact, this is a deterministic system. If we knew the rules of chemistry completely, and understood chemical reactivity fully, we should be able to answer this question and to predict the outcome of a reaction. Thus, we might use quantum mechanical calculations for exploring the structure and energetics of various transition states in order to find out which reaction pathway is followed. This requires calculations of quite a high degree of sophistication. In addition, modeling the influence of solvents on... [Pg.542]

With better hardware and software, more exact methods can be used for the representation of chemical structures and reactions. More and more quantum mechanical calculations can be utilized for chemoinformatics tasks. The representation of chemical structures will have to correspond more and more to our insight into theoretical chemistry, chemical bonding, and energetics. On the other hand, chemoinformatics methods should be used in theoretical chemistry. Why do we not yet have databases storing the results of quantum mechanical calculations. We are certain that the analysis of the results of quantum mechanical calculations by chemoinformatics methods could vastly increase our chemical insight and knowledge. [Pg.624]

Covers theory and applications of ah initio quantum mechanics calculations. The discussions are useful for understanding the differences between ah initio and semi-empirical methods. Although both sections are valuable, the discussion of the applications oi ah initio theory fills a void. It includes comparisons between experiment and many types and levels of calculation. The material is helpful in determining strategies for, and the validity of. ah initio calculations. [Pg.3]

Ohlaiii a new stable structure as a starting point for a single point, quantum mechanical calculation, which provides a large set ol structural and electronic properties. [Pg.57]

Quantum mechanical calculation of molecular dynamics trajectories can sim ulate bon d breakin g and frtrm ation.. Although you dt) n ot see th e appearance or disappearan ce ofhonds, you can plot the distan ce between two bonded atom s.. A distan ce excccdi n g a theoretical bond length suggests bond breaking. [Pg.90]

The IlyperChein log file includes calculated dipole moments of 111 oiccu les. To set th e am min t o f in form anon collected in th e log file, eh an gc the value of the Qu an turn Prin t Level set tin g in the eh em. in 1 File. Xote that the sign convention used in the quantum mechanical calculation of dipoles is opposite to that used in 111 oiccu lar mech an ics dipole calculation s this reflects th e differing sign conventions ofphysics and chemistry. [Pg.135]

In our treatment of molecular systems we first show how to determine the energy for a given iva efunction, and then demonstrate how to calculate the wavefunction for a specific nuclear geometry. In the most popular kind of quantum mechanical calculations performed on molecules each molecular spin orbital is expressed as a linear combination of atomic orhilals (the LCAO approach ). Thus each molecular orbital can be written as a summation of the following form ... [Pg.61]

See also in sourсe #XX -- [ Pg.67 , Pg.76 ]

See also in sourсe #XX -- [ Pg.187 ]

See also in sourсe #XX -- [ Pg.93 ]

See also in sourсe #XX -- [ Pg.2 , Pg.123 , Pg.165 ]

See also in sourсe #XX -- [ Pg.74 ]

See also in sourсe #XX -- [ Pg.43 , Pg.53 ]

See also in sourсe #XX -- [ Pg.95 , Pg.202 , Pg.242 ]

See also in sourсe #XX -- [ Pg.360 ]

See also in sourсe #XX -- [ Pg.91 , Pg.407 , Pg.412 ]

See also in sourсe #XX -- [ Pg.75 , Pg.117 ]

See also in sourсe #XX -- [ Pg.48 ]

See also in sourсe #XX -- [ Pg.144 , Pg.212 , Pg.265 ]

See also in sourсe #XX -- [ Pg.34 ]

See also in sourсe #XX -- [ Pg.148 , Pg.150 , Pg.162 ]

See also in sourсe #XX -- [ Pg.3 , Pg.9 , Pg.143 , Pg.165 , Pg.231 , Pg.241 , Pg.252 ]

See also in sourсe #XX -- [ Pg.45 ]

See also in sourсe #XX -- [ Pg.3 , Pg.2032 ]

See also in sourсe #XX -- [ Pg.148 ]

See also in sourсe #XX -- [ Pg.50 , Pg.179 ]

See also in sourсe #XX -- [ Pg.686 ]

See also in sourсe #XX -- [ Pg.114 ]

Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...