Hilltop

Department of Chemistry and Biochemistry, University of Maryland, Baltimore County Campus, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA... 

Another recently developed approach is the use of plastic containment capsules (modified Hilltop Chambers) for administration of well-measured doses in a moisturized microenvironment (Derelanko et al., 1987). 

Center for Molecular Design and Synthesis, Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea. Fax +82 42 869 5818 e-mail kimyh kaist.ac.kr Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel. Fax +972 8934 4142 e-mail edward.korshin weizmann.ac.il Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA. Fax +1 410 455 2608 e-mail jliebman umbc.edu... 

Hilltop Chambers (Hill Top Research, Miamiville, Ohio)... 

Fig. 2.13 The plausible stationary points on the propenol potential energy surface. A PES scan (Fig. 2.14) indicated that 1 is the global minimum and 4 is a relative minimum, while 2 and 3 are transition states and 5 and 6 are hilltops. AMI calculations gave relative energies for 1,2,3 and 4 of 0, 0.6, 14 and 6.5 kJ mol-1, respectively (5 and 6 were not optimized). The arrows represent one-step (rotation about one bond) conversion of one species into another...

The force constant matrix is called the Hessian.6 The Hessian is particularly important, not only for geometry optimization, but also for the characterization of stationary points as minima, transition states or hilltops, and for the calculation of IR spectra (Section 2.5). In the Hessian d2E/dqiq2 = d2E/dq2qi, as is true for all well-behaved functions, but this systematic notation is preferable the first subscript refers to the row and the second to the column. The geometry coordinate matrices for the initial and optimized structures are... 

Once a stationary point has been found by geometry optimization, it is usually desirable to check whether it is a minimum, a transition state, or a hilltop. This is done by calculating the vibrational frequencies. Such a calculation involves finding the normal-mode frequencies these are the simplest vibrations of the molecule,... 

A stationary point could of course be characterized just from the number of negative force constants, but the mass-weighting requires much less time than calculating the force constants, and the frequencies themselves are often wanted anyway, for example for comparison with experiment. In practice one usually checks the nature of a stationary point by calculating the frequencies and seeing how many imaginary frequencies are present a minimum has none, a transition state one, and a hilltop more than one. If one is seeking a particular transition state the criteria to be satisfied are ... 

Why are chemists but rarely interested in finding and characterizing second-order and higher saddle points (hilltops) ... 

Fig. 5.46 Cyclopropylamine conformations at the B3LYP/6-31G level. Structure 1 is a hilltop whose two imaginary frequencies indicate that it wants to undergo nitrogen pyramidalization and rotation about the C-N bond to form the transition states (nomenclature ts 1/2 connects 1 and 2, etc.) and, eventually, the minima. Each C, species has an enantiomer of the same energy...

A 7i-cyclopropane with the end methylene groups coplanar, as shown here, is (0,0)-trimethylene specifying the twist dihedral allows designation of the other conformers, e.g. (0,90)-trimethylene in which the putative 7i bond is completely broken [56]. The model chemistries, then, each led to two trimethylene stationary points HF to a hilltop and a transition state, MP2 to a transition state and a relative minimum, and B3LYP to two transition states. 

The short answer is, because they (the hilltops, not the chemists) don t do anything chemically. In a chemical reaction, we have (at least two) minima, and molecules move between them, passing through transition states, which are first order saddle points. Although in passing from one minimum to another all molecules do not strictly follow the intrinsic reaction coordinate (IRC) the lowest energy pathway on a PES that connects the minima, very few molecules are likely to stray so far outside the IRC that they pass through a hilltop [1],... 

Following a pre-treatment phase of female subjects with soap washing on the legs, baseline visual scaling scores were determined according to the following grades in Table 17.1. In the occlusion studies Hilltop chambers with 0.3 ml of test solutions or control were attached to the skin surface... 

No, in fact we were having an argument. Tired of.the park, she wanted us to go home, while I preferred to stay there with the hilltop view of London. My heart was beating a little faster, but I put it down to anxiety. 

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