Raman vs. IR Spectroscopy

Raman and Fourier-transform infared or FTIR spectroscopy offer molecular information about the structure and composition of chemical and biological samples. Because of the fundamental principles that govern each technology, both can yield complementary information. However, frequently one technology is a better choice, depending on the nature of the application.

a. Raman spectroscopy yields information about intra- and intermolecular vibrations. The former provides a spectrum characteristic of the specific vibrations of atoms in a molecule and is valuable for identifying a substance, form and molecular backbone configuration to name a few. The latter yields information about lower frequency modes, which reflects crystal lattice structure and polymorph form.

b. Infrared spectroscopy's greatest value lies in its ability to probe the “fingerprint region” of the spectrum where intramolecular vibrations are well-defined and highly characteristic of the bonding of atoms.

A practical example of differentiation for these two technologies is in the investigation of a crystallization process, in which Raman analyzes solid crystal form(s) and IR concurrently measures the solution-phase characteristics such as supersaturation.

Raman. The Raman effect is weak (often referred to as a one in ten million event), resulting from an energy event when photons of monochromatic light (laser) interact with a molecule that leads to a release of inelastic photons.

When photons interact with a molecule, an electron may be excited to a higher energy level (i.e. an excited virtual state) but not fully undergo an electronic transition. The electron then can relax to a vibrational energy level that is different than that of the incident energy. The difference between the energy of the incident photon and the energy of the scattered photon is called the Raman shift. The molecule's interaction with light can induce a shifting of the molecule's electron cloud that results in a change in polarizability. A molecule has specific energy transitions related to its intramolecular bonds in which a change of polarizability occurs, and these changes give rise to Raman active modes. The frequency of the Raman shift is related to the type of bond being excited and the intensity of this Raman peak is proportional to the number of those bonds.

IR. Infrared (IR) spectroscopy is a stronger technique that relies on the absorption of light by molecules at frequencies corresponding to the fundamental vibrations of those molecules. Molecules with functional groups that have strong dipoles display strong peaks in the IR, whereas functional groups that have weak dipoles and readily undergo a change in polarizability display strong peaks in Raman.

The motion of atoms in a molecule have a natural frequency of vibration related to their bond strength. When the frequency of a bond’s vibration is equal to the frequency of IR radiation directed onto it, the bond absorbs the radiation. The specific IR frequencies that a molecule's bonds absorb gives rise to an IR spectrum of the molecule.

Below we will outline the advantages and limitations of each technology along with reactions appropriate for each. Then we will explore how the instrumentation can provide value for key application areas and recent journal articles for further reading.

Raman and IR spectroscopy provide real-time profiling of reaction species and track reaction initiation, progress, transient intermediate(s) and end-point. They also allow rapid development of kinetics data and support proposed reaction mechanisms.

Both spectrometry methods can investigate catalytic bulk and surface structure, study adsorbed species on catalytic surfaces, examine ligand-metal bonding, develop structure-activity relationships and optimize catalytic processes by measuring the effect of reaction variables on product yield, quality and catalyst activity. All of this can be done via in-situ IR and Raman spectroscopy under actual pressure/temperature, as well as analyzing homogeneous catalytic reactions in flow.

For a wide range of reaction classes, use inline chemically inert flow cells (Raman) and ATR flow cells (IR) to continuously monitor and ensure the stability of flow reactions. IR and Raman technologies support rapid process optimization studies in flow by tracking key reaction species as a function of varying flow conditions.

In-situ IR measures solutes in the solution phase for crystallization development, and in-situ Raman spectroscopy measures crystal growth and polymorphism as a function of variables such as solvents, seeding, cooling profile, temperature and pH.

In-situ IR and Raman analysis can be utilized to support various downstream processing steps including primary capture, buffer exchange, purification, bioconjugation and formulation. In these applications and others, real-time analysis eliminates the traditional bottlenecks and delays associated with traditional offline, grab-sample analysis.

Raman spectroscopy is powerful for analyzing polymer backbone structure, chain length, ring opening and conformation. Use IR spectroscopy for analyzing side chains, terminal end groups of long-chain molecules and functional groups. Acquire data for reaction kinetics, monomer conversion rates, reactivity ratios, activation energies, the role of initiators, intermediates and by-product formation all in real time without the need for extractive sampling and offline analyses. In-situ analysis eliminates the irreproducibility of offline sampling caused by increased reaction viscosity and missed analysis data points during fast polymerizations, thereby providing more accurate kinetic data. 

The instrumentation and interface to the sample for these two techniques are similar in approach, but different in the details.

Raman spectrometers utilize a laser as the source (typically visible or near-IR laser), whereas IR spectrometers typically employ a black body radiator (such as a glow bar) to provide energy in the mid-infrared region.

Additionally, the optomechanical systems are different in that Raman spectrometers use a holographic grating with a highly sensitive CCD detector, and FTIR spectrometers use an interferometer to provide modulated radiation that is detected by pyroelectric (DTGS) or, more sensitive, photonic (MCT) detectors.

Both techniques use fiber optic probes for in-situ reaction monitoring. In the case of Raman, the fiber optic is composed of silica and can range in length from a few meters (2 m) to several (>50 m) meters or kilometers. IR fiber probes are constructed of material that transmit mid-IR radiation such as chalcogenide glasses or silver halide (AgX) and are limited in length (<4 m) due to natural attenuation of the IR energy by the fiber optic material. The interface between sample and probe in FTIR spectroscopy is an internal reflecting ATR sensor made of diamond or silicon crystal, whereas the interface to the sample for in-situ Raman measurements uses a sapphire lens to focus the laser at a spot near the sapphire lens/sample interface.

ReactIR and ReactRaman are part of an integrated family of products that are combined across the iC software platform to provide co-understanding of reaction kinetics, mechanism and pathway information for enhanced reaction characterizatoin.

ReactRaman is an exceptionally compact, compact, interchangeable probe or flow cell-based Raman system that is optimized for in-situ monitoring of chemical reactions and crystallization processes. The system uses a 785 nm solid state laser in order to reduce sample fluorescence while maintaining an excellent scattering cross-section to maximize the Raman signal. The laser energy is transmitted to and collected from the sample by fiber optic cables and a chemically hardened insertion probe equipped with a sapphire lens to interface with the sample. The scattered energy is passed onto a high performance holographic grating, and finally a CCD detector captures this energy to yield the Raman spectrum.

ReactIR is a compact FTIR system dedicated for chemical reaction monitoring with either an interchangeable insertion probe or flow cell that interfaces to batch or continuous flow syntheses, respectively. FTIR spectrometers in general consist of several key components – a light source (typically an infrared glow bar), an interferometer with both fixed and moving mirrors and a thermal or photonic detector. Broad band infrared energy from the source is directed onto a beamsplitter which passes the energy along two different paths. One path has a fixed mirror at the end, the other a moving mirror. The infrared energy from these two paths returns and recombines at the beam splitter causing a constructive and destructive interference pattern, the interferogram. This modulated infrared beam is passed to the sample where it is absorbed as a function of the molecular structure of the sample and therefore generates the IR spectrum for analysis.

ReactIR and ReactRaman merge the power of real-time spectroscopy with equivalent ease-of-use. The result: comprehensive reaction understanding for expert and non-expert users alike.

1. Compact and portable. Small, stackable units save valuable space in the fume hood or glove box and allow the flexibility to deploy in-situ spectroscopy wherever you need it most.

2. Plug in and ready-to-run. A combination of outstanding system performance and optimized probe integration provides stability and sensitivity – without any special requirements, such as liquid nitrogen.

3. One click analytics. Obtain high-quality information quickly with automated reaction profile and trending capabilities. Advanced yet intuitive software identifies and tracks key reaction species, including transient intermediates that affect product purity.

4. Integrated synthesis platform. Industry-standard iC Software allows complete workflow automation and integration of process and analytical data for synthesis, crystallization, and other unit operations.

5. Study all types of chemistries. A variety of batch and flow based options enable the study of chemistry across a wide range of conditions – even in aggressive reactants and reagents.

Ya Wang, Jun Yu, Yanding Wang, Zhuwen Chen, Lei Dong, Rongming Cai, Mei Hong, Xia Long and Shihe Yang, “in-situ templating synthesis of mesoporous Ni–Fe electrocatalyst for oxygen evolution reaction”, RSC Adv., (2020), 10, 23321.

The authors report using amorphous mesoporous fumed silica (MFS) as a framework on which to diffuse Ni²⁺ and Fe³⁺ ions in order to create a electrocatalyst for oxygen evolution reaction. In conjunction with x-ray photoelectron spectroscopy measurements, Raman spectroscopy measurements indicate the Ni²⁺ ions are covalently bonded to surface Si-OH groups whereas Fe-Si-O bonds are formed by the infusion of Fe³⁺ ions into the MFS framework.

In MFS, Raman bands at 345–450, 575, 750, 973 and 1070 cm^-1 are associated with various Si-O-Si vibrations and Si-OH stretching. When the Fe content is high, Raman bands at 332, 495 and 1163 cm^-1 appear and are assigned to Fe–O–Si species. For MFS with high Ni content, the Si-OH stretching band at 973 cm^-1 is decreased and no other new bands are present. This indicates that the Ni is bonded on the surface Si–OH, and not part of the MFS silica framework. The ability of Raman spectroscopy to measure lower frequency vibrations was important to obtain these bonding insight. 

Kallakuri Suparna Rao, Fredéric St-Jean, Archana Kumar, “Quantitation of a Ketone Enolization and a Vinyl Sulfonate Stereoisomer Formation Using Inline IR Spectroscopy and Modeling”, Org. Process Res. Dev. 2019, 23, 945−951.

The authors used ReactIR to investigate the formation of a tetrasubstituted acyclic olefin via ketone enolization and tosylation in a two-step, one-pot reaction. They successfully tracked the conversion of the ketone to an enolate by univariate modelling based on the ketone carbonyl band at 1685 cm^-1. The model was then used to follow the consumption rate of the starting material and then to define completion as a function of varying reaction conditions.

To determine the ratio of E versus Z tetrasubstituted vinyl tosylate isomeric products for the second step, the researchers constructed PCA and PLS multivariate models using the full spectrum in-situ IR and offline HPLC data. These models enabled the relative amount of the two isomers to be determined in subsequent reactions, and the models were validated with data from reactions obtained under varying conditions.

Wanya Li, Peng Shi, Lina Jia, Yanxiao Zhao, Binqiao Sun, Mingtao Zhang, Junbo Gong, Weiwei Tang, “Eutectics and Salt of Dapsone With Hydroxybenzoic Acids: Binary Phase Diagrams, Characterization and Evaluation”, J. Pharm. Sci., (2020), 109(7), 2224-2236.

The authors report the synthesis of the antibiotic dapsone with hydroxybenzoic acid coformers to form new multicomponent solids. The structure and characterization of these solids were determined with X-ray diffraction, DSC, IR, and Raman measurements, indicating that a new salt of DAP with 2,6-dihydroxybenzoic acid (26DHBA) and 4 eutectics with other hydroxybenzoic acids were formed. The IR and Raman measurements indicated that ionic interactions occur in the new salt.

The IR spectrum of a 1:1 physical mixture of DAP and 26DHBA is an additive spectrum, made up of the spectrum of the two individual compounds. The IR spectrum of the new DAP-26DHBA (1:1) product shows significant changes. As examples, the carbonyl frequency for 26DHBA at 1677 cm^-1 disappears, and a new band at 1386cm^-1 appears in the spectrum of the product , indicating the formation of an ionic salt. Spectral changes in the Raman measurements of the DAP-26DHBA (1:1) product are consistent with the IR results. Moreover, Raman spectra of DAP-SAL, DAP-3HBA, DAP-4HBA, and DAP25DHBA eutectics [salicylic acid (SAL), 3-hydroxybenzoic acid (3HBA), 4- hydroxybenzoic acid (4HBA), 2,5-dihydroxybenzoic acid (25DHBA), and 2,6-dihydroxybenzoic acid (26DHBA)] are consistent with the spectra of the respective individual components, indicating that the eutectics do not have the new crystalline phase present.

IR spectroscopy

• Si-minYu, William K. Snavely, Raghunath V. Chaudhari, Bala Subramaniam, “Butadiene hydroformylation to adipaldehyde with Rh-based catalysts: Insights into ligand effects”, Mol. Catalysis (2020), 484, 110721.
• Masahiro Hosoya, Shogo Nishijima, Noriyuki Kurose, “Management of the Heat of Reaction under Continuous Flow Conditions Using inline Monitoring Technologies”, Org. Process Res. Dev. 2020, 24, 6, 1095–1103.
• MarkoTrampuž, DušanTeslić, BlažLikozar, “Process analytical technology-based (PAT) model simulations of a combined cooling, seeded and antisolvent crystallization of an active pharmaceutical ingredient (API)”, Powder Technology (2020), 366, 873-890.
• K. Sateesh Reddy, Bandi Siva, S. Divya Reddy, N. Reddy Naresh, T. V. Pratap, B. Venkateswara Rao, Yi-An Hong, B. Vijaya Kumar, A. Krishnam Raju, P. Muralidhar Reddy, Anren Hu, “in-situ FTIR Spectroscopic Monitoring of the Formation of the Arene Diazonium Salts and Its Applications to the Heck–Matsuda Reaction”, Molecules (2020), 25, 2199.
• Patrick J. Morgan, Magnus W. D. Hanson-Heine, Hayden P. Thomas, Graham C. Saunders, Andrew C. Marr, Peter Licence, C–F Bond Activation of a Perfluorinated Ligand Leading to Nucleophilic Fluorination of an Organic Electrophile, Organometallics (2020), 39(11), 2116–2124.
• Dennis Svatunek, Gottfried Eilenberger, Christoph Denk, Daniel Lumpi, Christian Hametner, Gnter Allmaier, Hannes Mikula, “Live Monitoring of Strain-Promoted Azide Alkyne Cycloadditions in Complex Reaction Environments by Inline ATR-IR Spectroscopy”, Chem. Eur. J. (2020), 26, 1–5.
• Meng Shan-Shui, Lin Li-Rong, Luo Xiang, Lv Hao-Jun, Zhao Jun-Ling, Chan Albert S. C., "Aerobic oxidation of alcohols with air catalyzed by decacarbonyldimanganese" (2019) Green Chemistry (22).
• Rao Kallakuri Suparna, St-Jean Frederic, Kumar Archana; "Quantitation of Ketone Enolization and Vinyl Sulfonate Stereoisomer Formation using inline IR spectroscopy and Modeling" (2019) Org. Process Res. Dev. (2019), 23(5), 945-951.
• Cameron J. Brown, Thomas McGlone, Stephanie Yerdelen, Vijay Srirambhatla, Fraser Mabbott, Rajesh Gurung, Maria L. Briuglia, Bilal Ahmed, Hector Polyzois, John McGinty, Francesca Perciballi, Dimitris Fysikopoulos, Pól MacFhionnghaile, Humera Siddique, Vishal Raval, Tomás S. Harrington, Antony D. Vassileiou, Murray Robertson, Elke Prasad, Andrea Johnston, Blair Johnston, Alison Nordon, Jagjit S. Srai, Gavin Halbert, Joop H. ter Horst, Chris J. Price, Chris D. Rielly, Jan Sefcika, Alastair J. Florence, “Enabling precision manufacturing of active pharmaceutical ingredients: workflow for seeded cooling continuous crystallisations”, Mol. Syst. Des. Eng., (2018), 3, 518–549. 

Raman spectroscopy

• Ya Wang, Jun Yu, Yanding Wang, Zhuwen Chen, Lei Dong, Rongming Cai, Mei Hong, Xia Long and Shihe Yang, “in-situ templating synthesis of mesoporous Ni–Fe electrocatalyst for oxygen evolution reaction”, RSC Adv., (2020), 10, 23321.
• Yaohui Huang, Ling Zhou, Wenchao Yang, Yang Li, Yongfan Yang, Zaixiang Zhang, Chang Wang, Xia Zhang, Qiuxiang Yin, “Preparation of Theophylline-Benzoic Acid Cocrystal and online Monitoring of Cocrystallization Process in Solution by Raman Spectroscopy” Crystals (2019), 9, 329
• Christos Xiouras, Giuseppe Belletti, Raghunath Venkatramanan, Alison Nordon, Hugo Meekes, Elias Vlieg, Georgios D. Stefanidis, Joop H. Ter Horst, “Toward Continuous Deracemization via Racemic Crystal Transformation Monitored by in-situ Raman Spectroscopy”, Cryst. Growth Des. (2019), 19, 5858−5868

Raman and IR spectroscopy

• Marek Trojanowicz, “Flow Chemistry in Contemporary Chemical Sciences: A Real Variety of Its Applications”, Molecules (2020), 25, 1434.
• Wanya Li, Peng Shi, Lina Jia, Yanxiao Zhao, Binqiao Sun, Mingtao Zhang, Junbo Gong, Weiwei Tang, “Eutectics and Salt of Dapsone With Hydroxybenzoic Acids: Binary Phase Diagrams, Characterization and Evaluation”, J. Pharm. Sci., (2020), 109(7), 2224-2236.
• Chuntian Hu, Christopher J. Testa, Wei Wu, Khrystyna Shvedova, Dongying Erin Shen, Ridade Sayin, Bhakti S. Halkude, Federica Casati, Paul Hermant, Anjana Ramnath, Stephen C. Born, Bayan Takizawa, Thomas F. O’Connor, Xiaochuan Yang, Sukumar Ramanujam, Salvatore Mascia, “An automated modular assembly line for drugs in a miniaturized plant”, Chem. Commun., (2020), 56, 1026-1029.
• Shihao Zhang, Ling Zhou, Wenchao Yang, Chuang Xie, Zhao Wang, Baohong Hou, Hongxun Hao, Lina Zhou, Ying Bao, Qiuxiang Yin; An Investigation into the Morphology Evolution of Ethyl Vanillin with the Presence of a Polymer Additive”, Cryst. Growth Des. (2020), 20(3), 1609–1617.
• Xiaoyun Chen, Yang Cheng, Masashi Matsuba, Xianghuai Wang, Shuangbing Han, Mowbray Jordan, and Qing Zhu, “in-situ Monitoring of Heterogeneous Hydrosilylation Reactions Using Infrared and Raman Spectroscopy: Normalization Using Phase-Specific Internal Standards”, Applied Spectroscopy, (2019), 73(11), 1299-1307.
• Cameron T. Armstrong, Cailean Q. Pritchard, Daniel W. Cook, Mariam Ibrahim, Bimbisar K. Desai, Patrick J. Whitham, Brian J. Marquardt, Yizheng Chen, Jeremie T. Zoueu, Michael J. Bortner, Thomas D. Roper, “Continuous flow synthesis of a pharmaceutical intermediate: a computational fluid dynamics approach”, React. Chem. Eng., (2019), 4, 634 .
• Carmen Mei, Sasmit Deshmukh, James Cronin, Shuxin Cong, Daniel Chapman, Nicole Lazaris, Liliana Sampaleanu, Ulrich Schacht, Katherine Drolet-Vives, Moriam Ore, Sylvie Morin, Bruce Carpick, Matthew Balmer, Marina Kirkitadze,”Aluminum Phosphate Vaccine Adjuvant: Analysis of Composition and Size Using offline and inline Tools”, Computational and Structural Biotechnology Journal (2019) 17, 1184-1194.