Formaldehyde H2CO

When formaldehyde is subjected to suitable optical excitation it dissociates into H2 and CO. The process is thought to involve an excitation to the first excited singlet state followed by internal conversion to a highly excited vibrational state of the ground singlet state that dissociates according to the equation 

Some years ago Vance and the present author[68] made a study of this surface with the targeted correlation technique using a Dunning double-zeta basis[70] that, 

We show the results of calculations at the ST03G and 6-3 IG levels of the AO basis. Table 16.8 shows the orbitals used and the number of functions produced for each case. These statistics apply to each of the calculations we give. 

The important difference between the ST03G and 6-3 IG bases is the arrangement of orbitals on the CO fragment. In its ground state CO has an orbital configuration of 

The 5(7 function is best described as a nonbonding orbital located principally on the C atom. In Table 16.8 the 27t orbital is the virtual orbital from the ground state RHF treatment. The primed orbitals on H are the same as we have used before, but those on CO are based upon an ROHF n jt calculation of the first triplet state. The raw 5a, 5a, lit, and lit taken directly from the calculations will not work, however. Their overlaps are much too large for an S matrix of any size ( 2 or 3) to be considered nonsingular by standard 16-place accuracy calculations. Therefore, for each high-overlap pair the sum and difference were formed. These are orthogonal, and do not cause any problems. 

Along the x axis we have an orbital ip0 = a(px) + i(s), in which a and fi are the mixing coefficients in the hybridization. Since we want to construct three equivalent orbitals, each one must have of the C(2s) orbital. 

Recall that 2px, 2py, 2pz, and 2s all are orthogonal to each other. Thus, the condition for the normalization of ip0 is 

Formaldehyde has symmetry operations which place it in the point group C2V. The character table for C2V was given in Table 6-1. Since 0(py) transforms as b2 and 0(pz) as bx, the ground 

Because the components (X,Y,Z) of the dipole vector in C2V transformas (AuB Bj the transition Aj — A2 is orbitally forbidden. The transition — 3A2 is both orbitally forbidden and spin-forbidden. Although the transitions are theoretically forbidden, they are both observed but with small intensities. The xAj — aAi transition is found between 4000 and 3000 A, and 1A1 —- rAz is found between 3700 and 2300 A. We shall not discuss the mechanisms that make these transitions slightly allowed. The Aj — 1A2 transition, with a maximum around 3000 A, is a trademark of the carbonylgroup. 

The lowest-energy electron promotion is from the 2b2 non-bonding n orbital to the 2bx antibonding n orbital, giving the configuration 

Orbital promotions of this type give rise to states, such as the a and A states of formaldehyde, which are commonly referred to as nn states. In addition, transitions to such states, for example the a — X and A— X transitions of formaldehyde, are referred to colloquially as n — n or n-io-n. transitions. 

Because of the pyramidal shape in these excited states the orbitals and states may be reclassified according to the Cs point group (Table A.l in Appendix A). 

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For example, in formaldehyde, H2CO, one forms sp hybrids on the C atom on the O atom, either sp hybrids (with one p orbital "reserved" for use in forming the n and 7i orbitals and another p orbital to be used as a non-bonding orbital lying in the plane of the molecule) or sp hybrids (with the remaining p orbital reserved for the n and 7i orbitals) can be used. The H atoms use their 1 s orbitals since hybridization is not feasible for them. The C atom clearly uses its sp2 hybrids to form two CH and one CO a bondingantibonding orbital pairs. 

Formaldehyde (H2CO) H C=0 H Carbon has two bonded pairs + one double bond which is counted as one bonded pair Trigonal planar Trigonal planar ... 

Arts. (a) Urea is a product of animal metabolism and thiourea is its sulfur analog, (b) Both hydrogen atoms of their parent, formaldehyde, H2CO, have been replaced with amino groups, and there arc no C—C or C—H bonds left. 

Let us consider an example for the calculation of reactivity parameters of formaldehyde (H2CO) from DFT calculations (Table 12.1). 

Methyl alcohol is poisonous and is commonly used to denature ethyl alcohol. Methanol poisoning results from ingestion, inhalation of methanol vapors, or absorption through the skin. Methanol is transformed in the body to formaldehyde (H2CO) by the enzyme alcohol dehydrogenase. The formaldehyde is then metabolized to formic acid (HCOOH)... 

Because of the importance of carbonyl groups to the mechanism of condensation reactions, much of the assembly of either straight-chain or branched-carbon skeletons takes place between compounds in which the average oxidation state of the carbon atoms is similar to that in carbohydrates (or in formaldehyde, H2CO). The diversity of chemical reactions possible with compounds at this state of oxidation is a maximum, a fact that may explain why carbohydrates and closely related substances are major biosynthetic precursors and why the average state of oxidation of the carbon in... 

The detailed mechanism of the dissociation of formaldehyde (H2CO) has been investigated in molecular beam experiments. The reaction itself may appear quite simple... 

Among other reactions, it is worth mentioning the formation of hydrogen chloride in a reaction with methane, molecular hydrogen, or other constituents such as formaldehyde, H2CO, or hydrogen peroxide, H202. An example is... 

Methanal (formaldehyde), H2CO. There are 12 valence electrons altogether (C = 4,... 

Ideally, a single A (or B) coefficient would be appropriate for each element type such as one AH for all hydrogens. It has been our experience, however, that atoms of the same element often require different coefficients in different functional groups. There would be, for example, different AH s for H in H-O, H-NR2, H-C(sp2), H-C(sp3), etc. Other complications may involve separate cross terms for each of the atom i - atom j interactions. For a simple molecule such as formaldehyde (H2CO), the Ay cross terms are handled in WMIN with three AH, Ac and Ao coefficients that are used to form the six possible cross terms. But other programs, (e.g. DMAREL) may require cross terms that encompass the Ahh, AHo> AHc, ACc, ACo and Aoo possibilities. This is undoubtedly more flexible and possibly more accurate but does have the disadvantage of requiring the evaluation of many cross term coefficients. Of course, in cases with separate cross terms, one can always use the WMIN simplification of AHo = (AH A0) /2, etc. 

The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

In this article, we present applications of CASVB to chemical reactions the unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO [5], and a series of hydrogen exchange reactions, H2+X — H+HX (X-F, Cl, Br, and I). The method in this article is based on the occupation numbers of VB structures that are defined by the weights of the spin-paired functions in the CASVB functions, so that we could obtain a quantitative description of the nature of electronic structures and chemical bonds even during reactions. 

The movement of pairs of electrons is key to understanding chemical reactions that involve the making and breaking of bonds (Box 2.1). Nucleophilic and electrophilic centers in a molecular structure are recognized by examining that structure. Nucleophilic centers (Box 2.2) that bear an electron pair prominently in a Lewis structure are easy to spot. Electrophilic centers are frequently less so, especially when the electrophilic center is a carbon atom. For example, the carbon of formaldehyde (H2CO) has all four of its valences occupied and does not seem to have a valence available to form a new bond with anything. 

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AH2, where A is a first-row element, formaldehyde (H2CO), benzene and some regular octahedral transition metal complexes. 

Fig. 18. Lower rotational energy levels of formaldehyde, H2CO. The two nuclear spin...

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

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