Information from overtone spectra

Overtone spectra are due to deviations from harmonic vibrational motion and, compared to the fundamental ones, give valuable information on the harmonic frequencies coe, the anharmonicities Xe, the shape of the potential curve and hence on the dissociation energy, which is related to the former by 

In the case of adsorbed diatomics the influence of interaction on the constants of internuclear potential and the bonding force can be separately determined. But, in general overtones are relatively weak and therefore not always detected in surface vibrational spectroscopy. As has been already pointed out with nitrogen in one of the previous sections, homonuclear diatomics are especially suited probe molecules for exploring the details of interaction in the zeolite pore system due to their particular features. Their induced spectra 

Though, due to their induced-only dipole moment, overtone transitions of adsorbed homonuclear diatomics should be very weak. 

NIR studies have been applied also to heteronuclear diatomics like CO and NO [95-97]. On alkali and alkaline earth cations the results are essentially similar to those obtained with homonuclear probes. In the case of CO and NO strengthening of the internal bonding is observed, caused by the withdrawal of electron density from slightly antibonding molecular orbitals in correspondence with quantum chemical calculations [70,72]. In spite of the uncertain Do determination on cobalt and copper ion-exchanged zeolites A the CO dissociation energy turns out to be decreased, which may be understood by backdonation of electronic charge into the carbonyl tt orbital [97,98]. 

---

From a time-dependent point of view, recurrences in the probability of occupying the initially prepared state give rise to the fine structure in the overtone absorption spectrum. Though rudiments of these recurrences may be present in the short-time trajectory P n,t), chaotic classical motion destroys the longer time recurrences, which occur quantum mechanically. It is these latter recurrences which are needed to evaluate fine details in the absorption spectrum. Thus, the classical trajectory method may be limited to the evaluation of low-resolution absorption spectra. However, it should be pointed out that progress is being made in extracting information from systems with... 

Water is the most important chemical constituent of fruits and vegetables and water highly absorbs NIR radiation, so the NIR spectrum of such materials is dominated by water. Further, the NIR spectrum is essentially composed of a large set of overtones and combination bands. This, in combination with the complex chemical composition of a typical fruit or vegetable causes the NIR spectrum to be highly convoluted. Multivariate statistical techniques are required to extract the information about quality attributes which is buried in the NIR spectrum Developments in multivariate statistical techniques such as partial least squares (PLS) regression and principal component analysis (PCA) are then applied to extract the required information from such convoluted spectra (Cozzolino et al., 2006b McClure, 2003 Naes et al., 2004 Nicolai et al., 2007 ). 

In order to obtain meaningful information from near infrared spectroscopy one must construct a model, or training set, consisting of spectra collected from known samples, and mixtures of samples. This training set is necessary because the NIR spectra of most compounds contains many overlapping broad peaks that are overtones and combination bands of the fundamental peaks observable in a typical mid-IR spectra. If there is only one component to be measured, this is not an issue, but when monitoring a mixture of components, such as is encountered in a pharmaceutical blend, complex mathematical models need to be utilized to generate accurate analytical information from an NIR spectrum. 

Molecules vibrate at fundamental frequencies that are usually in the mid-infrared. Some overtone and combination transitions occur at shorter wavelengths. Because infrared photons have enough energy to excite rotational motions also, the ir spectrum of a gas consists of rovibrational bands in which each vibrational transition is accompanied by numerous simultaneous rotational transitions. In condensed phases the rotational stmcture is suppressed, but the vibrational frequencies remain highly specific, and information on the molecular environment can often be deduced from Unewidths, frequency shifts, and additional spectral stmcture owing to phonon (thermal acoustic mode) and lattice effects. 

The final two examples of the determination of excited state distortions are large bimetallic compounds whose electronic absorption spectra are broad and featureless. We must turn entirely to resonance Raman spectroscopy to measure the distortions because all of the information in the electronic spectrum is buried under the envelope. Fortunately, the resonance Raman profiles contain a great deal of information. These molecules were chosen as illustrative examples precisely because the resonance Raman spectra are so rich. The spectrum contains long overtone progressions and combination bands. Excitation profiles of not only the fundamentals but also of overtones and combination bands will be used to determine the distortions. The power of time-dependent theory from Section III.F and experimental examples of the effects of A on fundamentals, overtones, and combination bands are shown. The calculated distortions provide new insight about the orbitals involved in the electronic transition. 

A spectrum obtained for a sediment sample is rich in chemical information. However, the information is found in several overlapping combination bands and spectral overtones, rather than in distinct absorption peaks. The development of a reliable calibration model is therefore one of the most important steps to gain quantitative information on chemical compounds from the spectral response of a sample. This involves a thorough consideration of experimental design and multivariate calibration. 

NIR spectroscopy was used to quantify the cure reaction of MDEA-epoxy resins (E/A = 1.4) carried out at 72 and 160°C. Anew assignment for the secondary amine at 6580 cm was proposed, supported by a synthesized model compound. Two different spectral treatments were proposed. One was based on normalization at 4610 to 4620 cm while the another was based on the subtraction of the normalized spectrum of a cured sample. Quantitative results were obtained from absorbance measurements in the combination region, while spectral decompositions and area measurements were necessary in the overtone region. The resulting complementary information allowed calculation of conversion rates of epoxide and amine I, and concentrations of amine II, amine III, hydoxyl groups, and ether links [96]. 

---