Linear-bond

Contrary to the dioxetanone pathway, DeLuca and Dempsey (1970) proposed a mechanism of the bioluminescence reaction that involves a multiple linear bond cleavage of luciferin peroxide... 

However, the linear bond cleavage hypothesis of the firefly bioluminescence was made invalid in 1977. It was clearly shown by Shimomura et al. (1977) that one O atom of the CO2 produced is derived from molecular oxygen, not from the solvent water, using the same 180-labeling technique as used by DeLuca and Dempsey. The result was verified by Wannlund et al. (1978). Thus it was confirmed that the firefly bioluminescence reaction involves the dioxetanone pathway. Incidentally, there is currently no known bioluminescence system that involves a splitting of CO2 by the linear bond cleavage mechanism. 

Answer Linear bond angles all 180° each C atom is sp hybridized and forms one u-bond and one -ir-bond to each adjacent C or O atom.]... 

The infrared spectra exhibit the absorption bands corresponding to all these species. Adsorbed CO is detected as IR absorption bands around 2050 cm" for the linearly bonded species and 1870 cm" for the bridge-bonded species. The presence of CO2 is clearly indicated by the sharp band at 2345 cm", which appears at higher potentials the formation of ( CHO)jjj is evidenced by the band at 1690 cm, while that of ( COOHXj by weak absorption bands around 1720 cm (see Figs. 6 to 8). 

The Pb atom resides in the center of the D3 symmetric cavity formed by the P2phen ligands coordinated to the two trigonal Pd° centers. The Pd(l)—Pb(l) and Pd(2)—Pb(l) separations are nearly identical, and a Pb center is nearly linearly bonded in Pd(l)—Pb(l)—Pd(2), indicating that the lone pair is stereochemically inactive (see Figure 108). 

According to Steiner et al. [69], who studied the complex of /TCD with 1,4-butanediol, a monomer model of PTHF, the crystal packing in this complex is the cage type and isomorphous to that of the jS-CD hydrate, and the methylene chain vibrates in the cavity. However, in the /J-CD-PTHF complex, the polymer chain lays fixed inside a column formed by linearly bonded CD molecules. In agreement with this picture, CP/MAS NMR spectra showed the PTHF chain in the complex to be much less flexible than that in its mixture with the CD. 

However, a different view of the bonding in XeF2 is provided by a molecular orbital approach in which a p orbital on Xe combines with a p orbital from each F to form a three-center, four-electron linear bond. Actually, the three atomic orbitals form three molecular orbitals, but only the bonding and nonbonding orbitals are populated. This population of the orbitals places nonbonding electron density on the F atoms to give polar Xe-F bonds. 

The main purpose of this chapter is to review the development of hypervalent chalcogen chemistry after the book of "Chemistry of Hypervalent Compounds edited by Akiba (1999).5 Although several types of compounds may belong to hypervalent ions or molecules by the definition,3a such hypervalent chalcogen compounds (E = S, Se, and Te) will be mainly discussed here that contain CT-type linear bonds.15... 

The linear bond in 12 Br4 is analyzed by the 6c 8e model. Linear Br-Se-Br 3c-4e bonds in 1 Br2 interact mutually to give 12 Br4, which contains the linear alignment of the six Se2Br4 atoms. The linear bond in 12 Br6 is analyzed... 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

An elimination-addition mechanism has been suggested for the substitution of amino for chloro in chlorobenzene.95 The neutral and symmetrical intermediate is called benzyne although it can not contain an ordinary linear bond system like that of acetylene. It is not known whether it is a distorted acetylene, a triplet (diradical), or a zwitterion. 

Electrochemical infrared spectroscopy can be used on all kinds of electrodes and for all substances that are IR active. It is particularly useful for the identification of reaction intermediates, and has been used extensively for the elucidation of the mechanisms of technologically important reactions. A case in point is the oxidation of methanol on platinum, where linearly bonded = C = O (i.e., CO bonded to one Pt atom) has been identified as an intermediate Figs. 15.7 and 15.8 show EMIRS [6c] and IRRAS [8] spectra of this species. Near 2070 cm-1 the EMIRS spectrum shows the typical form produced by a peak that shifts with potential. This shift can be followed in the IRRAS spectrum... 

Figure 8.5 Transmission infrared spectra of CO adsorbed at room temperature on a series of silica-supported Pd-Ag alloys. CO adsorption on Ag is negligible. The dashed spectra are for higher CO exposures (0.5 Torr), the full spectra for low exposures (0.01 Torr). On pure Pd, CO adsorbs predominantly on multiple sites (frequen-cies below 2000 cm"1). As Pd becomes diluted with Ag, linearly bonded CO becomes dominant, indicating that ensembles of two or more Pd atoms to accommodate multiply bonded CO are no longer present, (from Soma-Noto and Sachtler [18]).

The cyanide ion was one of the first adsorbed ions which was found to be surface enhanced (8,9) on silver electrodes. In this first stage of SERS investigation of the cyanide ion, it was assumed to be linearly bonded to Ag, through the carbon atom, perpendicular to the metal surface (9). However faced with the pressure of explaining the SERS mechanism, alternative structures were proposed. With the conception of the ad-atom model for SERS, Otto conjectured that the main SERS band was due... 

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

There are no known exceptions to rule 2, though many fewer data are available. The sensitivity parameter is by definition obtainable only where the linear bond length-reactivity relationship is observed, so exceptions are in any case less likely. It is not readily accessible — for accurate definition it requires good quality structures for a series of at least four to five derivatives — so any use outside the area of crystal-structure correlation is likely to be limited to situations where a particularly important question of mechanism or reactivity cannot be resolved by conventional approaches.21... 

The theoretical treatment of the hydrophobic effect is limited to pure aqueous systems. To describe chromatographic separations in RPC Horvath and Melander developed the solvophobic theory [47]. In this theory, no special assumptions are made about the properties of solute and solvent, and besides hydrophobic interaction electrostatic and other specific interactions are included. The theory has been valuable to describe the retention of nonpolar [48], polar [49], and ionizable [50] solutes in RPC. The modulation of selectivity via secondary equilibria (variation of pH, ion pair formation [51]) can also be described. On the other hand, it is not a problem to find examples of dispersive interactions in literature, e.g., separation of carotinoids with a long chain (C30) RP gives a higher selectivity compared to standard RP C18 cyclohexanols are preferentially retarded on cyclohexyl-bonded phases compared to phases with linear-bonded alkyl groups. 

It was found that the N—O stretching frequency is potential dependent. For Rh(lll), it was observed that with increasing potential the bridge-bonded NO changes into linearly bonded species. 

A similar study was performed by the same group on a model catalyst system i.e., a Mn/Ru (0001) surface, prepared by sputtering Mn on a Ru (0001) surface at room temperature using a sputtered ion gun. The techniques of choice were SSIMS, EELS and TPD. It was concluded that Mn reduces the coverage of CO adsorbed on the surface by physically blocking the adsorption sites. The adsorbed CO molecule is predominantly linearly bonded, whereas EELS indicated a possible existence of an electronic interaction between the deposited Mn and Ru. This was reflected by the change in the CO stretching frequency to lower wavenumbers once Mn was deposited on the Ru surface. 

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