13.2 Principles of Molecular Spectroscopy: Quantized Energy State When examining Figure 13. 1 be sure to keep the following two relationships in mind 1. Frequency is inversely proportional to wavelength; the greater the frequency, the shorter the wavelength. 2. Energy is directly proportional to frequency; electromagnetic radiation of higher frequency possesses more energy than radiation of lower frequency Depending on its source, a photon can have a vast amount of energy; gamma rays and X-rays are streams of very high energy photons. Radio waves are of relatively lov energy. Ultraviolet radiation is of higher energy than the violet end of visible light Infrared radiation is of lower energy than the red end of visible light. When a molecule is exposed to electromagnetic radiation, it may absorb a photon, increasing its energy by an amount equal to the energy of the photon Molecules are highly selective with respect to the frequencies that they absorb. Only photons of certain specific frequencies are absorbed by a molecule. The particular photon energies absorbed by a molecule depend on molecular structure and can be measured with instruments called spectrometers. The data obtained are very sensitive indicators of molecular structure and have revolution ized the practice of chemical analysis. 13.2 PRINCIPLES OF MOLECULAR SPECTROSCOPY: QUANTIZED ENERGY STATES What determines whether or not a photon is absorbed by a molecule? The most impor tant requirement is that the energy of the photon must equal the energy difference between two states, such as two nuclear spin states, two vibrational states, or two elec- tronic states. In physics, the term for this is resonance--the transfer of energy between two objects that occurs when their frequencies are matched. In molecular spectroscopy. we are concerned with the transfer of energy from a photon to a molecule, but the idea is the same. Consider, for example, two energy states of a molecule designated El and E2 in Figure 13. 2. The energy difference between them is E2-El, or AE In nuclear magnetic resonance(NMR) spectroscopy these are two different spin states of an atomic nucleus; in infrared (IR)spectroscopy, they are two different vibrational energy states in ultraviolet-visible (UV-VIS) spectroscopy, they are two different electronic energy states. Unlike kinetic energy, which is continuous, meaning that all values of kinetic energy are available to a molecule, only certain energies are possible for electronic, vibra- tional, and nuclear spin states. These energy states are said to be quantized. More of the molecules exist in the lower energy state E than in the higher energy state E2. Exci tation of a molecule from a lower state to a higher one requires the addition of an incre- ment of energy equal to AE. Thus, when electromagnetic radiation is incident upon a molecule, only the frequency whose corresponding energy equals AE is absorbed. All other frequencies are transmitted Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment △E=E,-E1=hy containing the sample through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compared with that at the source. When the frequency is reached at which the sample absorbs radiation, the El detector senses a decrease in intensity. The relation between frequency and absorption is plotted on a strip chart and is called a spectrum. A spectrum consists of a series of peaks FIGURE 13.2 Two energy at particular frequencies; its interpretation can provide structural information. Each type states of a molecul of spectroscopy developed independently of the others, and so the format followed in tion of energy presenting the data is different for each one. An NMR spectrum looks different from an from its lower ener egplecul IR spectrum, and both look different from a UV-VIs spectrum to the next higher state Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteWhen examining Figure 13.1 be sure to keep the following two relationships in mind: 1. Frequency is inversely proportional to wavelength; the greater the frequency, the shorter the wavelength. 2. Energy is directly proportional to frequency; electromagnetic radiation of higher frequency possesses more energy than radiation of lower frequency. Depending on its source, a photon can have a vast amount of energy; gamma rays and X-rays are streams of very high energy photons. Radio waves are of relatively low energy. Ultraviolet radiation is of higher energy than the violet end of visible light. Infrared radiation is of lower energy than the red end of visible light. When a molecule is exposed to electromagnetic radiation, it may absorb a photon, increasing its energy by an amount equal to the energy of the photon. Molecules are highly selective with respect to the frequencies that they absorb. Only photons of certain specific frequencies are absorbed by a molecule. The particular photon energies absorbed by a molecule depend on molecular structure and can be measured with instruments called spectrometers. The data obtained are very sensitive indicators of molecular structure and have revolution￾ized the practice of chemical analysis. 13.2 PRINCIPLES OF MOLECULAR SPECTROSCOPY: QUANTIZED ENERGY STATES What determines whether or not a photon is absorbed by a molecule? The most impor￾tant requirement is that the energy of the photon must equal the energy difference between two states, such as two nuclear spin states, two vibrational states, or two elec￾tronic states. In physics, the term for this is resonance—the transfer of energy between two objects that occurs when their frequencies are matched. In molecular spectroscopy, we are concerned with the transfer of energy from a photon to a molecule, but the idea is the same. Consider, for example, two energy states of a molecule designated E1 and E2 in Figure 13.2. The energy difference between them is E2 E1, or E. In nuclear magnetic resonance (NMR) spectroscopy these are two different spin states of an atomic nucleus; in infrared (IR) spectroscopy, they are two different vibrational energy states; in ultraviolet-visible (UV-VIS) spectroscopy, they are two different electronic energy states. Unlike kinetic energy, which is continuous, meaning that all values of kinetic energy are available to a molecule, only certain energies are possible for electronic, vibra￾tional, and nuclear spin states. These energy states are said to be quantized. More of the molecules exist in the lower energy state E1 than in the higher energy state E2. Exci￾tation of a molecule from a lower state to a higher one requires the addition of an incre￾ment of energy equal to E. Thus, when electromagnetic radiation is incident upon a molecule, only the frequency whose corresponding energy equals E is absorbed. All other frequencies are transmitted. Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sample through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compared with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted on a strip chart and is called a spectrum. A spectrum consists of a series of peaks at particular frequencies; its interpretation can provide structural information. Each type of spectroscopy developed independently of the others, and so the format followed in presenting the data is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 13.2 Principles of Molecular Spectroscopy: Quantized Energy States 489 E2 E1 E E2 E1 h FIGURE 13.2 Two energy states of a molecule. Absorp￾tion of energy equal to E2 E1 excites a molecule from its lower energy state to the next higher state. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website