Infrared Diffusion‐Ordered Spectroscopy Reveals Molecular Size and Structure**

Abstract

Inspired by ideas from NMR, we have developed Infrared Diffusion-Ordered Spectroscopy (IR-DOSY), which simultaneously characterizes molecular structure and size. We rely on the fact that the diffusion coefficient of a molecule is determined by its size through the Stokes-Einstein relation, and achieve sensitivity to the diffusion coefficient by creating a concentration gradient and tracking its equilibration in an IR-frequency resolved manner. Analogous to NMR-DOSY, a two-dimensional IR-DOSY spectrum has IR frequency along one axis and diffusion coefficient (or equivalently, size) along the other, so the chemical structure and the size of a compound are characterized simultaneously. In an IR-DOSY spectrum of a mixture, molecules with different sizes are nicely separated into distinct sets of IR peaks. Extending this idea to higher dimensions, we also perform 3D-IR-DOSY, in which we combine the conformation sensitivity of femtosecond multi-dimensional IR spectroscopy with size sensitivity.

Raman Diffusion-Ordered Spectroscopy

“…Using these two diffusion coefficients, we globally fit the full time-dependent Raman data set at all frequencies to create a Raman-DOSY spectrum. The time- and frequency-dependent Raman intensity S measured at the far edge of the channel is fitted to the function S ( ν , t ) = prefix∑ i = 1 N S i ( ν ) C ( D i t / L 2 ) where N is the number of species in the sample (in this case N = 2), S i (ν) is the Raman spectrum of species number i , D i is the diffusion coefficient of species i , and C (τ) is a function (given in eq ) depending only on the reduced time τ = Dt / L 2 . From the fit, we obtain the spectra S 1,2 (ν) of the two compounds in the mixed solution.…”

“…at the edge of the channel), this is given by c ( t ) = C ( Dt / L 2 ) = C (τ) with C (τ) a function that does not depend on D or L . Full summation expressions for C can be found in ref , but for practical purposes, we can achieve a relative precision of 10 –8 (with respect to the exact solution) at all times using the expression C ( τ ) = { .25ex2ex 0 if .25em τ = 0 1 2 − 1 2 ∑ n = − 3 3 false( prefix− 1 false) n .25em erf ( n + 1 / 2 2 τ ) if .25em 0 < τ < 0.2 1 2 − 2 π normale − π 2 τ if .25em τ ≥ 0.2 where erf( x ) is the Gaussian error function.…”

“…However, the Raman spectrum generally provides little information about the size of molecules or aggregates. To address this issue, we took the experience that we gained from our previous work on infrared-based diffusion ordered spectroscopy (IR-DOSY) , and designed a Raman-based DOSY method. Similar to an NMR-DOSY spectrum, a Raman-DOSY spectrum is a two-dimensional spectrum with Raman frequency on one axis and diffusion coefficient on the other and thus provides simultaneous information on the chemical structure and the size of a compound or of a mixture of compounds.…”

Molecular Weight Determination of Relevant Organometallic Catalysts in Solution by DOSY-NMR to Monitor Reactivity

“…23 Promising results are provided by the recently presented infrared DOSY which could be a useful addition to DOSY-NMR. 24 However, since the ECCs have been established from light atom molecules, mainly hydrocarbons, the use of this method currently is restricted largely to molecules not containing atoms heavier than second-row elements. Determination of MW of molecules containing heavier atoms like bromine with these "standard" ECCs tends to fail.…”

“…Recently, progress has been made to apply the ECC method to investigate natural products involved in various hydrogen bonds . Promising results are provided by the recently presented infrared DOSY which could be a useful addition to DOSY-NMR . However, since the ECCs have been established from light atom molecules, mainly hydrocarbons, the use of this method currently is restricted largely to molecules not containing atoms heavier than second-row elements.…”