Enthalpies of formation of cis-azobenzene and trans-azobenzene

All organic compounds contain carbon. Most contain hydrogen. This brief study discusses the enthalpy of formation of a collection of organic compounds containing only oxygen or nitrogen accompanying the carbon.By definition, organic chemistry is the chemistry of carboncontaining compounds. Almost always, in practice organic chemistry is also the chemistry of compounds containing hydrogen. Relatedly, organic thermochemistry discusses the thermochemistry of carbon-, and most often, hydrogen-, containing species. There are several thousand organic species for which the enthalpies of formation is known. In the current note we discuss the enthalpies of formation of those species that lack hydrogen, in particular, carbon oxides and carbon nitrides but not halocarbons. The current study will be limited to those species for which these data are found from experiment, whether or not the species be isolable. We will also limit our attention to neutral species and so ignore all ions such as the commonplace CN -and rather exotic CN + . We proceed first through the oxides and then the nitrides of carbon. Within each group, the compounds are discussed in order of increasing number of carbons, and then by increasing number of oxygens or nitrogens. All unreferenced enthalpies of formation in the current study are taken from Ref.[1] and may tacitly be assumed to refer to gaseous species at 298 K and 1 atm unless otherwise said.While there are a respectable number of species containing only carbon and oxygen, there are only three isolable compounds for which enthalpy of formation data are available: carbon monoxide (CO), carbon dioxide (CO 2 ) and propadienedione (C 3 O 2 ), the so-called carbon suboxide. Not limiting our species of interest by grounds of isolability allows us to consider other oxides of carbon, CO 3 , C 2 O, and C 3 O.Enthalpies of formation of CO and CO 2 are found from numerous sources. We choose the values -110.525 and -393.509 kJ/mol [2]. These values are well-established, the latter providing a benchmark value for the enthalpies of formation and combustion of nearly all organic compounds. Perhaps the only thermochemical observation of consequence we will make in this study relates to their carbon-oxygen bond energies. The ratio of the C-O bond energy in CO 2 to that of (the mean) value for CO is very nearly the same as the ratio for the Si-O bond energies in SiO 2 and gaseous SiO despite the fact that CO 2 is ''naturally'' a gas while SiO 2 is ''naturally'' a solid. Perhaps this is not so strange: this near constant ratio in the gas phase for some other pairs of species containing group 14-16 atoms was first pointed out in Ref. [3], and extended by consideration of experimentally unexplored neutrals [4] as well as valence isoelectronic pairs of cations [5], anions [5], and some hydrogen-containing pentatomics as well [6]. Summarizing, for 10 valence electron AB and 16 valence electron AB 2 , we findThis is a comfortably simple approximation for bond energies and a good example of how the investigation of

show abstract

“…[29] and [30], and a value of 345 kJ/mol for liquid phenyl azide from Ref. [31] we find this reaction is exothermic by ca.…”

mentioning

confidence: 66%

Paradigms and paradoxes: organic thermochemistry without hydrogen: carbon oxides and nitrides

Slayden

Liebman

2008

Struct Chem

View full text Add to dashboard Cite

show abstract

“…6 In solution, calculations have shown that the free volume required for isomerization is 0.12 nm 3 for inversion or 0.25 nm 3 for rotation, 22 but, to our knowledge, no consideration of the volume for solid isomerization has been undertaken. Given the previously mentioned volume considerations and the energy gap between the two isomers (49.1 kJ·mol −1 ), 38 it would not be expected that mechanical pressure could force isomerization from tAB to cAB, because although the molecular volumes of the trans and cis isomers are similar, the volume of the transition state required to convert between them is significantly larger. 20 Successive measurements were made at increasing pressures up to 4 GPa using a 514 nm probe laser.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Azobenzene Photoisomerization under High External Pressures: Testing the Strength of a Light-Activated Molecular Muscle

et al. 2012

View full text Add to dashboard Cite

The photoinduced isomerization and thermal back relaxation of an azobenzene-functionalized polymer poly(disperse red 1 acrylate) were investigated at increasing external pressures up to 1.5 GPa inside a diamond-anvil spectroscopic cell. The thermal cis-trans isomerization was monitored by laser pump-probe spectroscopy, which demonstrated an increase in the half-life of the isomerization process with increasing pressure. Additionally, the cis content of the photostationary state gradually decreased as a function of pressure, with complete arrest of the trans-cis photoisomerization above 1.5 GPa. The fact that the photoswitching behavior however could still be observed beyond 1 GPa is remarkable and is effectively a measure of the strength of the azobenzene chromophore as an artificial muscle. The changes in the Raman shifts of both trans- and cis-azobenzene were also investigated from ambient pressure up to 4 GPa, and no discontinuities were observed in the pressure vs wavenumber plots indicating no change in phase. The cis-trans photoisomerization of azobenzene was shown however to still be inducible at all the pressures investigated, confirming the suitability of these molecules for high-efficiency light actuation.

show abstract

“…The trans state is more stable in the dark by approximately 10 kcal mol À1 [42]. The trans and cis isomers have distinct but overlapping absorption spectra [43].…”

Section: Synthetic Photoswitchesmentioning

confidence: 99%