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Conformational Isomer
In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as conformations. Conformers/rotamers differ little in their energies, so they are almost never separable in a practical sense. Rotations about single bonds are subject to small energy barriers. When the time scale for interconversion is long enough for isolation of individual rotamers (usually arbitrarily defined as a half-life of interconversion of 1000 seconds or longer), the species are termed atropisomers (''see:'' atropisomerism). The ring-flip of substituted cyclohexanes constitutes a common form of conformers. The study of the energetics of bond rotation is referred to as conformational analysis. In some cases, conformational analysis can be used to predict and explain product selectivity, mechanisms, and rates of reactions. Conform ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Periodic Function
A periodic function, also called a periodic waveform (or simply periodic wave), is a function that repeats its values at regular intervals or periods. The repeatable part of the function or waveform is called a ''cycle''. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to describe oscillations, waves, and other phenomena that exhibit periodicity. Any function that is not periodic is called ''aperiodic''. Definition A function is said to be periodic if, for some nonzero constant , it is the case that :f(x+P) = f(x) for all values of in the domain. A nonzero constant for which this is the case is called a period of the function. If there exists a least positive constant with this property, it is called the fundamental period (also primitive period, basic period, or prime period.) Often, "the" period of a function is used to mean its fundamental period. A funct ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Atropisomerism
Atropisomers are stereoisomers arising because of hindered rotation about a covalent bond, single bond, where Gibbs free energy, energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual rotamers. They occur naturally and are of occasional importance in pharmaceutical design. When the substituents are achiral, these conformers are enantiomers (''atropoenantiomers''), showing axial chirality; otherwise they are diastereomers (''atropodiastereomers''). Etymology and history The word ''atropisomer'' (, , meaning "not to be turned") was coined in application to a theoretical concept by German biochemist Richard Kuhn for Karl Freudenberg's seminal ''Stereochemie'' volume in 1933. Atropisomerism was first experimentally detected in a tetra substituted biphenyl, a diacid, by George Christie and James Kenner in 1922. Michinori Ōki further refined the definition of atropisomers taking into account th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Allylic Strain
250 px, Allylic strain in an olefin. Allylic strain (also known as A1,3 strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin (a synonym for an alkene) with an allylic substituent on the other end.Eric V. Anslyn and Dennis A. Dougherty ''Modern Physical Organic Chemistry'' University Science Books, 2006. If the substituents (R and R') are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other. Allylic strain was first recognized in the literature in 1965 by Johnson and Malhotra. The authors were investigating cyclohexane conformations including endocyclic and exocylic double bonds when they noticed certain conformations were disfavored due to the geometry constraints caused by the double bond. Organic chemists capitalize on the rigidity resulting from allylic strain for use in asymmetric reactions. Q ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carbohydrate Conformation
Carbohydrate conformation refers to the overall three-dimensional structure adopted by a carbohydrate (saccharide) molecule as a result of the through-bond and through-space physical forces it experiences arising from its molecular structure. The physical forces that dictate the three-dimensional shapes of all molecules—here, of all monosaccharide, oligosaccharide, and polysaccharide molecules—are sometimes summarily captured by such terms as "steric interactions" and "stereoelectronic effects" (see below). Saccharide and other chemical conformations can be reasonably shown using two-dimensional structure representations that follow set conventions; these capture for a trained viewer an understanding of the three-dimensional structure via structure drawings (see organic chemistry article, and "3D Representations" section in molecular geometry article); they are also represented by stereoscopy, stereograms on the two dimensional page, and increasingly using 3D display technologi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrocycles
Macrocycles are often described as molecules and ions containing a Ring (chemistry), ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. Synthesis The formation of macrocycles by ring-closure is called macrocyclization. The central challenge to macrocyclization is that ring-closing reactions do not favor the formation of large rings. Instead, medium sized rings or polymers tend to form. Early macrocyclizations were achieved ketonic decarboxylations for the preparation of terpenoid macrocycles. So, while Ružička was able to produce various macrocycles, the yields were low. This kinetic problem can be addressed by using high-dilution reactions, whereby intramolecular processes are favored relative to polymerizations. Reactions amenable to high dilution include Dieckmann condensation and related based-induced reactions of esters with remote halides. So ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cycloalkane
In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the ring (chemistry), monocyclic Saturated and unsaturated compounds, saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring (possibly with side chains), and all of the carbon-carbon bonds are single bond, single. The larger cycloalkanes, with more than 20 carbon atoms are typically called ''cycloparaffins''. All cycloalkanes are isomers of alkenes. The cycloalkanes without side chains (also known as monocycloalkanes) are classified as small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cyclotridecane), and large (all the rest). Besides this standard definition by IUPAC, the International Union of Pure and Applied Chemistry (IUPAC), in some authors' usage the term ''cycloalkane'' includes also those saturated hydrocarbons th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cyclohexane Conformation
Cyclohexane conformations are any of several three-dimensional shapes adopted by cyclohexane. Because many compounds feature structurally similar six-membered rings, the structure and dynamics of cyclohexane are important prototypes of a wide range of compounds. The internal angles of a regular, flat hexagon are 120°, while the preferred angle between successive bonds in a carbon chain is about 109.5°, the tetrahedral angle (the arc cosine of −). Therefore, the cyclohexane ring tends to assume non-planar (warped) conformations, which have all angles closer to 109.5° and therefore a lower strain energy than the flat hexagonal shape. Consider the carbon atoms numbered from 1 to 6 around the ring. If we hold carbon atoms 1, 2, and 3 stationary, with the correct bond lengths and the tetrahedral angle between the two bonds, and then continue by adding carbon atoms 4, 5, and 6 with the correct bond length and the tetrahedral angle, we can vary the three dihedral angles f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Klyne–Prelog System
In stereochemistry, the Klyne–Prelog system (named for William Klyne and Vladimir Prelog) for describing conformations about a single bond offers a more systematic means to unambiguously name complex structures, where the torsional or dihedral angles are not found to occur in 60° increments. Klyne notation views the placement of the substituent on the front atom as being in regions of space called anti/syn and clinal/periplanar relative to a reference group on the rear atom. A plus (+) or minus (−) sign is placed at the front to indicate the sign of the dihedral angle. Anti or syn indicates the substituents are on opposite sides or the same side, respectively. Clinal substituents are found within 30° of either side of a dihedral angle of 60° (from 30° to 90°), 120° (90°–150°), 240° (210°–270°), or 300° (270°–330°). Periplanar substituents are found within 30° of either 0° (330°–30°) or 180° (150°–210°). Juxtaposing the designations produces the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Approximate Potential Function For The Conformational Analysis Of Unbranched Linear Alkanes With Even-numbered Chains
An approximation is anything that is intentionally similar but not exactly equal to something else. Etymology and usage The word ''approximation'' is derived from Latin ''approximatus'', from ''proximus'' meaning ''very near'' and the prefix ''ad-'' (''ad-'' before ''p'' becomes ap- by assimilation) meaning ''to''. Words like ''approximate'', ''approximately'' and ''approximation'' are used especially in technical or scientific contexts. In everyday English, words such as ''roughly'' or ''around'' are used with a similar meaning. It is often found abbreviated as ''approx.'' The term can be applied to various properties (e.g., value, quantity, image, description) that are nearly, but not exactly correct; similar, but not exactly the same (e.g., the approximate time was 10 o'clock). Although approximation is most often applied to numbers, it is also frequently applied to such things as mathematical functions, shapes, and physical laws. In science, approximation can refer to u ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Steric Effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape (conformational isomerism, conformation) and chemical reaction, reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel. Steric hindrance Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested in ''intermolecular reactions'', whereas discussion of steric effects often focus on ''intramolecular interactions''. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions. Steric hindran ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Harmonic
In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st harmonic''; the other harmonics are known as ''higher harmonics''. As all harmonics are periodic at the fundamental frequency, the sum of harmonics is also periodic at that frequency. The set of harmonics forms a '' harmonic series''. The term is employed in various disciplines, including music, physics, acoustics, electronic power transmission, radio technology, and other fields. For example, if the fundamental frequency is 50 Hz, a common AC power supply frequency, the frequencies of the first three higher harmonics are 100 Hz (2nd harmonic), 150 Hz (3rd harmonic), 200 Hz (4th harmonic) and any addition of waves with these frequencies is periodic at 50 Hz. In music, harmonics are used on string instruments and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
