A general expression for vibrational Hamiltonians expressed in oblique coordinates.

We examine the properties of oblique coordinates. The coordinates, introduced by Zúñiga et al. [J. Phys. B: At., Mol. Opt. Phys. 52, 055101, (2019)], reduce vibrational mode-mixing and enhance the quality of vibrational assignments in quantum mechanical investigations of two-dimensional model Hamiltonians. Oblique coordinates are obtained by making non-orthogonal rotations of the original coordinates that convert the matrix representation of the quadratic Hamiltonian operator into a block-diagonal matrix where the blocks are distinguished by the total quanta of vibrational excitation. Using techniques for the polar decomposition of matrices, we present a scheme for finding these coordinates for systems of arbitrary dimensions. Several molecular examples are presented that highlight the advantages of these coordinates.

Investigating Intramolecular H Atom Transfer Dynamics in ß-Diketones with Ultrafast Infrared Spectroscopies and Theoretical Modeling.

The vibrational signatures and ultrafast dynamics of the intramolecular H-bond in a series of ß-diketones are investigated with 2D IR spectroscopy and computational modeling. The chosen ß-diketones exhibit a range of H atom donor-acceptor distances and asymmetry along the H atom transfer coordinate that tunes the intramolecular H-bond strength. The species with the strongest H-bonds are calculated to have very soft H atom potentials, resulting in highly red-shifted OH stretch fundamental frequencies and dislocation of the H atom upon vibrational excitation. These soft potentials lead to significant coupling to the other normal mode coordinates and give rise to the very broad vibrational signatures observed experimentally. The 2D IR spectra in both the OH and OD stretch regions of the light and deuterated isotopologues reveal broadened and long-lived ground-state bleach signatures of the vibrationally hot molecules. Polarization-sensitive transient absorption measurements in the OH and OD stretch regions reveal notable isotopic differences in orientational dynamics. Orientational relaxation was measured to occur on ∼600 fs and ∼2 ps time scales for the light and deuterated isotopologues, respectively. The orientational dynamics are interpreted in terms of activated H/D atom transfer events driven by collective intramolecular structural rearrangements.

Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K+- and Ba2+-Benzocryptand Complexes.

UV photofragment spectroscopy and IR-UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional [2.2.2]-benzocryptand cage to the incorporation of a single K+ or Ba2+ imbedded inside it (labeled as K+-BzCrypt, Ba2+-BzCrypt). We studied the isolated ion-cryptand complex under cryo-cooled conditions, brought into the gas phase by nano-electrospray ionization. Incorporation of a phenyl ring in place of the central ethyl group in one of the three N-CH2-CH2-O-CH2-CH2-O-CH2-CH2-N chains provides a UV chromophore whose S0-S1 transition we probe. K+-BzCrypt and Ba2+-BzCrypt have their S0-S1 origin transitions at 35,925 and 36,446 cm-1, respectively, blue-shifted by 174 and 695 cm-1 from that of 1,2-dimethoxybenzene. These origins are used to excite a single conformation of each complex selectively and record their IR spectra using IR-UV dip spectroscopy. The alkyl CH stretch region (2800-3000 cm-1) is surprisingly sensitive to the presence and nature of the encapsulated ion. We carried out an exhaustive conformational search of cage conformations for K+-BzCrypt and Ba2+-BzCrypt, identifying two conformations (A and B) that lie below all others in energy. We extend our local mode anharmonic model of the CH stretch region to these strongly bound ion-cage complexes to predict conformation-specific alkyl CH stretch spectra, obtaining quantitative agreement with experiment for conformer A, the gas-phase global minimum. The large electrostatic effect of the charge on the O- and N-lone pairs affects the local mode frequencies of the CH2 groups adjacent to these atoms. The localized CH2 scissors modes are pushed up in frequency by the adjacent O/N-atoms so that their overtones have little effect on the alkyl CH stretch region. However, the localized CH2 wags are nearly degenerate and strongly coupled to one another, producing an array of delocalized wag normal modes, whose highest frequency members reach up above 1400 cm-1. As such, their overtones mix significantly with the CH stretch modes, most notably involving the CH2 symmetric stretch fundamentals of the central ethyl groups in the all-alkyl chains and the CH stretches adjacent to the N-atoms and antiperiplanar to the nitrogen lone pair.

High energy vibrational excitations of nitromethane in liquid water.

The pathways and timescales of vibrational energy flow in nitromethane are investigated in both gas and condensed phases using classical molecular mechanics, with a particular focus on relaxation in liquid water. We monitor the flow of excess energy deposited in vibrational modes of nitromethane into the surrounding solvent. A marked energy flux anisotropy is found when nitromethane is immersed in liquid water, with a preferential flow to those water molecules in contact to the nitro group. The factors that permit such anisotropic energy relaxation are discussed, along with the potential implications on the molecule's non-equilibrium dynamics. In addition, the energy flux analysis allows us to identify the solvent motions responsible for the uptake of solute energy, confirming the crucial role of water librations. Finally, we also show that no anisotropic vibrational energy relaxation occurs when nitromethane is surrounded by argon gas.

Unraveling the Vibrational Spectral Signatures of a Dislocated H Atom in Model Proton-Coupled Electron Transfer Dyad Systems.

Phenol-benzimidazole and phenol-pyridine proton-coupled electron transfer (PCET) dyad systems are computationally investigated to resolve the origins of the asymmetrically broadened H-bonded OH stretch transitions that have been previously reported using cryogenic ion vibrational spectroscopy in the ground electronic state. Two-dimensional (2D) potentials describing the strongly shared H atom are predicted to be very shallow along the H atom transfer coordinate, enabling dislocation of the H atom between the donor and acceptor groups upon excitation of the OH vibrational modes. These soft H atom potentials result in strong coupling between the OH modes, which exhibit significant bend-stretch mixing, and a large number of normal mode coordinates. Vibrational spectra are calculated using a Hamiltonian that linearly and quadratically couples the H atom potentials to over two dozen of the most strongly coupled normal modes treated at the harmonic level. The calculated vibrational spectra qualitatively reproduce the asymmetric shape and breadth of the experimentally observed bands in the 2300-3000 cm-1 range. Interestingly, these transitions fall well above the predicted OH stretch fundamentals, which are computed to be surprisingly red-shifted (<2000 cm-1). Time-dependent calculations predict rapid (<100 fs) relaxation of the excited OH modes and instant response from the lower-frequency normal modes, corroborating the strong coupling predicted by the model Hamiltonian. The results highlight a unique broadening mechanism and complicated anharmonic effects present within these biologically relevant PCET model systems.

Modeling Anharmonic Effects in the Vibrational Spectra of High-Frequency Modes.

High-resolution vibrational spectra of C-H, O-H, and N-H stretches depend on both molecular conformation and environment as well as provide a window into the frequencies of many other vibrational degrees of freedom as a result of mode mixing. We review current theoretical strategies that are being deployed to both aid and guide the analysis of the data that are encoded in these spectra. The goal is to enhance the power of vibrational spectroscopy as a tool for probing conformational preferences, hydrogen bonding effects away from equilibrium, and energy flow pathways. Recent years have seen an explosion of new methods and strategies for solving the nuclear Schrödinger equation. Rather than attempt a comprehensive review, this work highlights specific molecular systems that we have chosen as representing bonding motifs that are important to chemistry and biology. We focus on the choices theoretical chemists make regarding the level of electronic structure theory, the representation of the potential energy surface, the selection of coordinates, preferences in basis sets, and methods of solution.

Correction: Single-conformation spectroscopy of cold, protonated DPG-containing peptides: switching ß-turn types and formation of a sequential type II/II' double ß-turn.

Correction for 'Single-conformation spectroscopy of cold, protonated DPG-containing peptides: switching ß-turn types and formation of a sequential type II/II' double ß-turn' by John T. Lawler et al., Phys. Chem. Chem. Phys., 2022, 24, 2095-2109, https://doi.org/10.1039/D1CP04852J.

A local mode study of ring puckering effects in the infrared spectra of cyclopentane.

We report and interpret recently recorded high-resolution infrared spectra for the fundamentals of the CH2 scissors and CH stretches of gas phase cyclopentane at -26.1 and -50 °C, respectively. We extend previous theoretical studies of this molecule, which is known to undergo barrierless pseudorotation due to ring puckering, by constructing local mode Hamiltonians of the stretching and scissor vibrations for which the frequencies, couplings, and linear dipoles are calculated as functions of the pseudorotation angle using B3LYP/6-311++(d,p) and MP2/cc-pVTZ levels of theory. Symmetrization (D5h) of the vibrational basis sets leads to simple vibration/pseudorotation Hamiltonians whose solutions lead to good agreement with the experiment at medium resolution, but which miss interesting line fractionation when compared to the high-resolution spectra. In contrast to the scissor motion, pseudorotation leads to significant state mixing of the CH stretches, which themselves are Fermi coupled to the scissor overtones.

Single-conformation spectroscopy of cold, protonated DPG-containing peptides: switching ß-turn types and formation of a sequential type II/II' double ß-turn.

D-Proline (DPro, DP) is widely utilized to form ß-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a DPG sub-unit that forms a ß-turn. To observe whether DPG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated DP and LP diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm-1) and amide I/II (1400-1800 cm-1) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm-1 region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N-H bending fundamentals of the NH3+ group in these protonated peptides. The model Hamiltonian accounts for experiment in quantitative detail. In the DP diastereomer, all the population is funneled into a single conformer which presented as a type II ß-turn with A and DP in the i + 1 and i + 2 positions, respectively. This structure was not the anticipated type II' ß-turn across DPG that we had hypothesized based on solution-phase propensities. Analysis of the conformational energy landscape shows that both steric and charge-induced effects play a role in the preferred formation of the type II ß-turn. In contrast, the LP isomer forms three conformations with very different structures, none of which were type II/II' ß-turns, confirming that LPG is not a ß-turn former. Finally, single-conformation spectroscopy was also carried out on the extended peptide [YAADPGAAA + H]+ to determine whether moving the protonated N-terminus further from DPG would lead to ß-hairpin formation. Despite funneling its entire population into a single peptide backbone structure, the assigned structure is not a ß-hairpin, but a concatenated type II/type II' double ß-turn that displaces the peptide backbone laterally by about 7.5 Å, but leaves the backbone oriented in its original direction.

Spectroscopic Manifestations of Indirect Vibrational State Mixing: Novel Anharmonic Effects on a Prereactive H Atom Transfer Surface.

The NH stretch region of the IR spectrum of methyl anthranilate is modeled in the S1 state to understand the connection between the absence of this fundamental in the fluorescence-dip infrared spectra of Blodgett et al. [Phys. Chem. Chem. Phys. 2020, 22, 14077] and its relevance to the H atom dislocation that occurs upon electronic excitation. A set of coordinates are chosen that highlight the role of certain low-frequency modes. A Hamiltonian is developed in which a large-amplitude two-dimensional surface describing the H-bonded H atom is linearly and quadratically coupled to the remaining degrees of freedom which are treated at the harmonic level. The surface is calculated within the time-dependent density functional theory framework by using the B3LYP/6-311++(d, p) level of theory with dispersion. Our spectral results show that indirect couplings lead to massive intensity sharing over hundreds of wavenumbers. This sharing is predicted to be dramatically reduced upon deuteration. The spectral broadening mechanism is found to involve off-resonant doorway states that are themselves strongly coupled to states nearly degenerate with the NH stretch fundamental and represents a complementary mechanism to previous explanations based on Fermi resonance or the presence of Franck-Condon like combination bands with low-frequency motions. Consistent with the spectra predictions, time-dependent calculations show that if the NH stretch fundamental were excited with an ultrafast laser, it would decay within 40 fs. The competition between H atom dislocation and vibrational relaxation is discussed.

A phase diagram for energy flow-limited reactivity.

Intramolecular energy flow (also known as intramolecular vibrational redistribution or IVR) is often assumed in Rice-Ramsperger-Kassel-Marcus, transition state, collisional energy transfer, and other rate calculations not to be an impediment to reaction. In contrast, experimental spectroscopy, computational results, and models based on Anderson localization have shown that ergodicity is achieved rather slowly during molecular energy flow. The statistical assumption in rate theories might easily fail due to quantum localization. Here, we develop a simple model for the interplay of IVR and energy transfer and simulate the model with near-exact quantum dynamics for a 10-degree of freedom system composed of two five-mode molecular fragments. The calculations are facilitated by applying the van Vleck transformation to local random matrix models of the vibrational Hamiltonian. We find that there is a rather sharp "phase transition" as a function of molecular anharmonicity "a" between a region of facile energy transfer and a region limited by IVR and incomplete accessibility of the state space (classically, the phase space). The very narrow transition range of the order parameter a happens to lie right in the middle of the range expected for molecular torsion, bending, and stretching vibrations, thus demonstrating that reactive energy transfer dynamics several kBT above the thermal energy occurs not far from the localization boundary, with implications for controllability of reactions.

The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database.

Revisiting recently published Raman jet spectra of monomeric formic acid with accurate high order perturbative calculations based on two explicitly correlated coupled-cluster quality potential energy surfaces from the literature, we assign and add 11 new vibrational band centers to the trans-HCOOH database and 53 for its three deuterated isotopologs. Profiting from the synergy between accurate calculations and symmetry information from depolarized Raman spectra, we reassign eight literature IR bands up to 4000 cm-1. Experimental detection of highly excited torsional states (ν9) of trans-HCOOH, such as 4ν9 and ν6 + 2ν9, reveals substantial involvement of the C-O stretch ν6 into the O-H bend/torsion resonance ν5/2ν9, which is part of a larger resonance polyad. Depolarization and isotopic C-D substitution experiments further elucidate the nature of Raman peaks in the vicinity of the O-H stretching fundamental (ν1), which seem to be members of a large set of interacting states that can be identified and described with a polyad quantum number and that gain intensity via resonance mixing with ν1.

The missing NH stretch fundamental in S1 methyl anthranilate: IR-UV double resonance experiments and local mode theory.

The infrared spectra of jet-cooled methyl anthranilate (MA) and the MA-H2O complex are reported in both S0 and S1 states, recorded using fluorescence-dip infrared (FDIR) spectroscopy under jet-cooled conditions. Using a combination of local mode CH stretch modeling and scaled harmonic vibrational character, a near-complete assignment of the infrared spectra is possible over the 1400-3700 cm-1 region. While the NH stretch fundamentals are easily observed in the S0 spectrum, in the S1 state, the hydrogen bonded NH stretch shift is not readily apparent. Scaled harmonic calculations predict this fundamental at just below 2900 cm-1 with an intensity around 400 km mol-1. However, the experimental spectrum shows no evidence of this transition. A local mode theory is developed in which the NH stretch vibration is treated adiabatically. Minimizing the energy of the corresponding stretch state with one quantum of excitation leads to a dislocation of the H atom where there is equal sharing between N and O atoms. The sharing occurs as a result of significant molecular arrangement due to strong coupling of this NH stretch to other internal degrees of freedom and in particular to the contiguous HNC bend. A two-dimensional model of the coupling between the NH stretch and this bend highlights important nonlinear effects that are not captured by low order vibrational perturbation theory. In particular, the model predicts a dramatic dilution of the NH stretch oscillator strength over many transitions spread over more than 1000 cm-1, making it difficult to observe experimentally.

Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state.

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-H2O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S0-S1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm-1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NHO[double bond, length as m-dash]C H-bond, which shifts the 3nπ* state well above the 1ππ* S1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck-Condon activity due to three in-plane vibrations that modulate the distance between the NH2 and CO2Me groups, ν33 (421 cm-1), ν34 (366 cm-1), and ν36 (179 cm-1). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NHO[double bond, length as m-dash]C H-bond. This leads to a shortening of the NHO[double bond, length as m-dash]C H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the HO distance compared to full H-atom transfer. As a result, the excited state process near the S1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.

Modeling vibrational anharmonicity in infrared spectra of high frequency vibrations of polyatomic molecules.

In this perspective, we review the challenges of calculating spectra of high-frequency XH vibrations (where X = C, N, or O) of molecules and small clusters. These modes are often coupled to nearly degenerate overtone and combination bands, greatly complicating the interpretation of the spectra. When molecules or clusters contain multiple XH groups, assigning spectra is difficult, especially when multiple conformers are present. We consider approaches appropriate for addressing these difficulties, focusing on systems with more than 15 atoms. At this size, the densities of states of these fundamentals are sufficiently high that it is not possible to calculate eigenstate-resolved spectra. Nonetheless, combining perturbation theory, empirical scalings of vibrational frequencies, and variational treatments of reduced dimensional Hamiltonians, one can identify and model the vibrational coupling pathways that influence observed spectral features. We describe how these methods have evolved through time as electronic structure methods and computational resources have advanced.

Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes.

An ultraviolet-infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S1 origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S1 origin fluorescence, resulting in spectra with high signal-to-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (Ac-Phe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch-bend Fermi resonance.

Infrared absorption spectra of partially deuterated methoxy radicals CH2DO and CHD2O isolated in solid para-hydrogen.

The investigation of partially deuterated methoxy radicals is important because the symmetry lowering from C3v to Cs provides new insights into the couplings between rovibronic states via Jahn-Teller and spin-orbit interactions. The vibrational spectrum of the partially deuterated methoxy radical CH2DO in a matrix of p-H2 has been recorded. This species was prepared by irradiating a p-H2 matrix containing deuterated d1-nitritomethane (CH2DONO) at 3.3 K with laser light at 355 nm. The identification of the radical is based on the photochemical behavior of the precursor and comparison of observed vibrational wavenumbers and infrared (IR) intensities with those predicted from a refined quartic, curvilinear, internal coordinate force field calculated with the coupled-cluster singles and doubles with perturbative triples/cc-pVTZ method. CH2DO reacts with H2 with a rate coefficient (3.5 ± 1.0) × 10-3 s-1. Predominantly c-CHDOH and a negligibly small amount of t-CHDOH were produced. This stereoselectivity results from the reaction H + Cs-CH2DOH, which was demonstrated by an additional experiment on irradiation of a CH2DOH/Cl2/p-H2 matrix with ultraviolet and IR light to induce the H + CH2DOH reaction; only c-CHDOH was observed from this experiment. Even though the energies of transition states and products for the formation of c-CHDOH and t-CHDOH differ by only ∼10 cm-1, the selective formation of c-CHDOH can be explained by tunneling of the hydrogen atom via an optimal tunneling path. Similarly, the vibronic spectrum for the partially deuterated specie d2-methoxy radical (CHD2O) was obtained upon irradiation of d2-nitritomethane (CHD2ONO) at 355 nm. Lines associated with the fundamental vibrational modes were observed and assigned; line positions agree with theoretically predicted vibrational wavenumbers. CHD2O reacts with H2 with a rate coefficient (6.0 ± 1.4) × 10-3 s-1; CD2OH was produced as a major product because the barrier for the formation of CHDOH from H + CHD2OH is greater by ∼400 cm-1. Rate coefficients of the decays of CH3O, CH2DO, CHD2O, and CD3O and their corresponding potential energy surfaces are compared.

Fingerprints of inter- and intramolecular hydrogen bonding in saligenin-water clusters revealed by mid- and far-infrared spectroscopy.

Saligenin (2-(hydroxymethyl)phenol) exhibits both strong and weak intramolecular electrostatic interactions. The bonds that result from these interactions compete with intermolecular hydrogen bonds once saligenin binds to one or more water molecules. Infrared (IR) ultraviolet (UV) ion-dip spectroscopy was used to study isolated saligenin-(H2O)n clusters (n = 1-3) in the far- and mid-IR regions of the spectrum. Both harmonic and anharmonic (coupled local modes and Born-Oppenheimer molecular dynamics) quantum chemical calculations were applied to assign cluster geometries to the measured spectra, and to assign vibrational modes to all spectral features measured for each cluster. The hydrated clusters with n = 1 and 2 have geometries that are quite similar to benzyl alcohol-water clusters, whereas the larger clusters with n = 3 show structures equivalent to the isolated water pentamer. Systematic shifts in the frequencies of three hydrogen bond (H-bond) deforming modes, namely OH stretching, OH torsion and H-bond stretching, were studied as a function of the hydrogen bond strength represented by either the OH bond length or the H-bond length. The shifts of the frequencies of these three modes correlate linearly to the OH length, despite both intra- and intermolecular H-bonds being included in this analysis. The OH torsion vibration displays the largest frequency shift when H-bonded, followed by the OH stretching vibrations and finally the H-bond stretching frequency. The frequency shifts of these H-bond deforming modes behave non-linearly as a function of the H-bond length, asymptotically approaching the frequency expected for the non H-bonded modes. The nonlinear behavior was quantified using exponential functions.

Assigning the low lying vibronic states of CH3O and CD3O.

The assignment of lines in vibrational spectra in strongly mixing systems is considered. Several low lying vibrational states of the ground electronic X∼2E state of the CH3O and CD3O radicals are assigned. Jahn-Teller, spin-orbit, and Fermi couplings mix the normal mode states. The mixing complicates the assignment of the infrared spectra using a zero-order normal mode representation. Alternative zero-order representations, which include specific Jahn-Teller couplings, are explored. These representations allow for definitive assignments. In many instances it is possible to plot the wavefunctions on which the assignments are based. The plots, which are shown in the adiabatic representation, allow one to visualize the effects of various higher order couplings. The plots also enable one to visualize the conical seam and its effect on the wavefunctions. The first and the second order Jahn-Teller couplings in the rocking motion dominate the spectral features in CH3O, while first order and modulated first order couplings dominate the spectral features in CD3O. The methods described here are general and can be applied to other Jahn-Teller systems.

Infrared laser spectroscopy of the n-propyl and i-propyl radicals: Stretch-bend Fermi coupling in the alkyl CH stretch region.

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite [CH3(CH2)3ONO] and i-butyl nitrite [(CH3)2CHCH2ONO], respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the CH stretching region. Several previously unreported bands are observed between 2800 and 3150 cm-1. The CH stretching modes observed above 3000 cm-1 are in excellent agreement with CCSD(T) anharmonic frequencies computed using second-order vibrational perturbation theory. However, between 2800 and 3000 cm-1, the spectra of n- and i-propyl radicals become congested and difficult to assign due to the presence of multiple anharmonic resonance polyads. To model the spectrally congested region, Fermi and Darling-Dennison resonances are treated explicitly using "dressed" Hamiltonians and CCSD(T) quartic force fields in the normal mode representation, and the agreement with experiment is less than satisfactory. Computations employing local mode effective Hamiltonians reveal the origin of the spectral congestion to be strong coupling between the high frequency CH stretching modes and the lower frequency CHn bending/scissoring motions. The most significant coupling is between stretches and bends localized on the same CH2/CH3 group. Spectral simulations using the local mode approach are in excellent agreement with experiment.