Intrinsic Reaction Coordinate Calculation

The intrinsic reaction coordinate (IRC) is an important concept in the study of chemical reactions in quantum chemistry, providing a unique connection from a given transition structure to local minima on the reactant and product sides, describing the optimal trajectory of the chemical process in terms of structural changes without considering thermal motion factors. This allows complex multi-step mechanisms to be easily understood as a simple set of basic reaction steps.

Alfa Chemistry is committed to providing chemical researchers with comprehensive and accurate chemical reaction IRC calculation services. IRC calculations are the most conclusive method for confirming that the correct transition state has been discovered and have been frequently used for quantum chemical research and prediction of chemical reaction pathways.

Our Service

Alfa Chemistry mainly uses quantum chemistry and molecular mechanics to calculate reaction transition states. We use calculation software 'Gaussian09/16' to provide the following high-quality and fast services:

IRC calculations require initial force constants to be performed. You must somehow provide these to Alfa Chemistry for calculation. Or customers can choose our additional transition state calculation service to obtain the structure of the transition state and the hessian (force constants) for that transition state. By default only the energy and reaction coordinates are reported for each point on the path; if geometric parameters along the path are required, let us know in advance.

IRC Calculation Methods:

Gaussian09/16 supports a variety of methods for generating IRC calculations, the three main ones are as follows.

• Local Quadratic Approximation (LQA): This is a very traditional IRC generation algorithm that requires the use of Hessian matrices at each step.
• Hessian-Based Predictor-Corrector (HPC): This is the default algorithm for Gaussian09/16. This method produces each point of the IRC by using the LQA method as a prediction step and a modified Bulrisch-Stoer method as a correction step. This combination of "prediction + correction" results in better accuracy than using only LQA.
• Other methods such as DVV, EulerPC, etc. are also supported in Gaussian09/16.

Fig 1. Intrinsic reaction coordinate (IRC) for the dyotropic interconversion of D / L -1,2-diphenyl-1,2-dibromoethane, computed at the B3LYP/6-311g level with a continuum solvent correction for benzene. (Braddock C, et al. 2012)

Applications

• The IRC can be used to confirm whether the structure of the transition state connects reactants and products in the study of chemical reaction mechanisms when it is sometimes challenging to determine whether the transition state connects reactants and products from the vibrational direction of the imaginary frequency alone.
• The transition state can also be verified via the IRC. The IRC starts at the transition stage and effectively moves downward along the path with the least amount of energy on either side. When the energy path flattens out, or when the reactant or product state is reached, the IRC comes to an end. Structures close to the reactants and products are obtained at each end. If the structure at the end of the IRC is not what we expect, there is a problem with the TS.
• IRC calculations can be performed by plotting the variation of a particular bond length, bond angle, or dihedral angle with the reaction coordinates.
• Alfa Chemistry may additionally record every structure (and structural parameter) in the IRC as a dynamic structure map, providing a more thorough understanding of the structural alterations in the transition state.

IRC calculations provide an efficient way to optimize chemical reaction processes. Our personalised full service will meet your innovative learning needs. If you are interested in our services, please feel free to contact us. We are happy to work with you and witness your success!

Reference

• Braddock C, et al. (2012). "Verification of Stereospecific Dyotropic Racemisation of Enantiopure D and L-1,2-Dibromo-1,2-Diphenylethane in Non-Polar Media." Chemical Communications. 48(71): 8943-8945.