# Tag Info

11

This is a task suited to single crystal XRD. This does require you to be able to form a crystal of your substance which can be difficult at times but this allows for potentially high accuracy of bond lengths/angles to be determined. In the spirit of this community though, if you do know the bonding structure from something like NMR, you could model the ...

11

A partial list of bond lengths I have determined from IR spectra over my career: \begin{array}{ccc} \rm{Molecule} & \rm{Bond ~Length ~ (picometers)} & \rm{References}\\ \hline \ce{Li_2}(1^1\Sigma_g^+) & 267.298 74(19)& \href{}\textrm{2009 JCP, 2013 PRA} \\ \ce{Li_2}(1^3\Sigma_u^+) & 417.000 6(32)& \href{}\textrm{2011 JMS, 2013 PRA (2)}...

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Wiberg (1968) Let's start with the "classic" bond order paper by Ken Wiberg (born in 1927 and still alive!). The Wiberg Bond Index (WBI) between fragments A and B of AB is calculated as follows: $$\tag{1} W_{AB} \equiv \sum_{\mu \in A}\sum_{\nu \in B}D_{\mu \nu}^2~ ,$$ where $D$ is the following density matrix: \begin{...

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I agree with all answers provided so far: you cannot quantitatively deduce bond lengths from infrared spectra. However, see the answer by Nike Dattani about the inverse, predicting IR spectra from theory. IR (and Raman) spectra can be very useful tools to understand properties associated with bond lengths. An example I really like comes from the high ...

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It depends on what you want from it. If you're an organic chemist, you'd be mostly interested in the type of bond (single, double, triple, aromatic), and for that the bond order is most relevant (but tricky to compute). I sent a paper away earlier this year which deals with the choice of bond order analysis, and will post a link to it when it will be online. ...

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Addendum: While the method described below is commonly used by experimentalists, I was mistaken that IR spectrum could not be used to obtain bond lengths. Nike's answer does a great job explaining how the entire potential energy surface can be determined by fitting using some model calculations. The important thing to learn from all this is that what you can ...

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To start with, we do have: LUCIA, a Correlation Program, 2019, developed by Kähler and Olsen. Nonorthogonal internally contracted multi-configurational perturbation theory and Dynamic correlation for nonorthogonal reference states. Q-CHEM. GronOR: Massively parallel and GPU- accelerated non-orthogonal configuration interaction for large molecular systems. ...

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Why Bond Order? Bond order isn't terribly useful to a computationalist directly; however, it can be invaluable for translating Quantum Mechanical results into a framework thats readily understood by experimentalists. Experimental chemists tend to make predictions about material properties, reaction feasibility, molecular structure, etc. based on a ...

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How to test bond order methods for chemical consistency Some of the bond order methods that claim to work do not give consistent results across different SZ values of a spin multiplet or different levels of theory (i.e., basis sets and exchange-correlation functionals). Consequently, there have been several published methods claimed to compute bond orders ...

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Laplacian Bond Order This method is an extension of the QTAIM (Quantum Theory of Atoms In Molecules) concept of using the Laplacian of the electron density $\nabla^2\rho$ to characterize bonding. Standard QTAIM just uses the value of $\nabla^2\rho$ at bond critical points (minimum electron density along the bond, max along orthogonal directions), but these ...

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There is more than one way to answer this question well. I'll give one answer here. Let's treat the two atoms (that will form a covalent bond) as two localized election states (orbitals). The detailed shape isn't important. The Hamiltonian of this system could be written:  H = V (n_{\uparrow,1} n_{\downarrow,1}+ n_{\uparrow,2} n_{\downarrow,2}) - t \sum \...

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I am surprised no-one above mentions the classic textbook example of the IR spectrum of HCl gas, which shows rather beautiful rotational structure superimposed on the vibrational band. The spacing of the rotational peaks, of course gives direct access to the angular momentum and the bond length. I say "directly" and "of course" - but you ...

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