Hexbyte Glen Cove Team measures the breakup of a single chemical bond thumbnail

Hexbyte Glen Cove Team measures the breakup of a single chemical bond

Hexbyte Glen Cove

Researchers measured the mechanical forces applied to break a bond between carbon monoxide and iron phthalocyanine, which appears as a symmetrical cross in scanning probe microscope images taken before and after the bond rupture. Credit: Pengcheng Chen et al.

The team used a high-resolution atomic force microscope (AFM) operating in a controlled environment at Princeton’s Imaging and Analysis Center. The AFM probe, whose tip ends in a single copper atom, was moved gradually closer to the iron-carbon bond until it was ruptured. The researchers measured the mechanical forces applied at the moment of breakage, which was visible in an image captured by the microscope. A team from Princeton University, the University of Texas-Austin and ExxonMobil reported the results in a paper published Sept. 24 in Nature Communications.

“It’s an incredible image—being able to actually see a single small molecule on a surface with another one bonded to it is amazing,” said coauthor Craig Arnold, the Susan Dod Brown Professor of Mechanical and Aerospace Engineering and director of the Princeton Institute for the Science and Technology of Materials (PRISM).

“The fact that we could characterize that particular , both by pulling on it and pushing on it, allows us to understand a lot more about the nature of these kinds of bonds—their strength, how they interact—and this has all sorts of implications, particularly for catalysis, where you have a molecule on a surface and then something interacts with it and causes it to break apart,” said Arnold.

Nan Yao, a principal investigator of the study and the director of Princeton’s Imaging and Analysis Center, noted that the experiments also revealed insights into how bond breaking affects a catalyst’s interactions with the surface on which it’s adsorbed. Improving the design of chemical catalysts has relevance for biochemistry, materials science and energy technologies, added Yao, who is also a professor of the practice and senior research scholar in PRISM.

In the experiments, the carbon atom was part of a carbon monoxide molecule and the iron atom was from iron phthalocyanine, a common pigment and chemical catalyst. Iron phthalocyanine is structured like a symmetrical cross, with a single iron atom at the center of a complex of nitrogen- and carbon-based connected rings. The iron atom interacts with the carbon of carbon monoxide, and the iron and carbon share a pair of electrons in a type of covalent bond known as a dative bond.

Yao and his colleagues used the atomic-scale probe tip of the AFM instrument to break the iron-carbon bond by precisely controlling the distance between the tip and the bonded molecules, down to increments of 5 picometers (5 billionths of a millimeter). The breakage occurred when the tip was 30 picometers above the molecules—a distance that corresponds to about one-sixth the width of a carbon atom. At this height, half of the iron phthalocyanine molecule became blurrier in the AFM image, indicating the rupture point of the chemical bond.

The researchers used a type of AFM known as non-contact, in which the microscope’s tip does not directly contact the molecules being studied, but instead uses changes in the frequency of fine-scale vibrations to construct an image of the molecules’ surface.

By measuring these frequency shifts, the researchers were also able to calculate the force needed to break the bond. A standard copper probe tip broke the iron-carbon bond with an attractive force of 150 piconewtons. With another carbon monoxide molecule attached to the tip, the bond was broken by a repulsive force of 220 piconewtons. To delve into the basis for these differences, the team used quantum simulation methods to model changes in the densities of electrons during .

The work takes advantage of AFM technology first advanced in 2009 to visualize single chemical bonds. The controlled breaking of a chemical bond using an AFM system has been more challenging than similar studies on bond formation.

“It is a great challenge to improve our understanding of how chemical reactions can be carried out by atom manipulation, that is, with a tip of a scanning probe microscope,” said Leo Gross, who leads the Atom and Molecule Manipulation research group at IBM Research in Zurich, and was the lead author of the 2009 study that first resolved the chemical structure of a molecule by AFM.

By breaking a particular bond with different tips that use two different mechanisms, the new study contributes to “improving our understanding and control of bond cleavage by atom manipulation. It adds to our toolbox for chemistry by atom manipulation and represents a step forward toward fabricating designed molecules of increasing complexity,” added Gross, who was not involved in the study.

The experiments are acutely sensitive to external vibrations and other confounding factors. The Imaging and Analysis Center’s specialized AFM instrument is housed in a high-vacuum environment, and the materials are cooled to a temperature of 4 Kelvin, just a few degrees above absolute zero, using liquid helium. These controlled conditions yield precise measurements by ensuring that the ‘ energy states and interactions are affected only by the experimental manipulations.

“You need a very good, clean system because this reaction could be very complicated—with so many involved, you might not know which bond you break at such a small scale,” said Yao. “The design of this system simplified the whole process and clarified the unknown” in breaking a chemical bond, he said.

The study’s lead authors were Pengcheng Chen, an associate research scholar at PRISM, and Dingxin Fan, a Ph.D. student at the University of Texas-Austin. In addition to Yao, other corresponding authors were Yunlong Zhang of ExxonMobil Research and Engineering Company in Annandale, New Jersey, and James R. Chelikowsky, a professor at UT Austin. Besides Arnold, other Princeton coauthors were Annabella Selloni, the David B. Jones Professor of Chemistry, and Emily Carter, the Gerhard R. Andlinger ’52 Professor in Energy and the Environment. Other coauthors from ExxonMobil were David Dankworth and Steven Rucker.

More information:
Breaking a dative bond with mechanical forces, Nature Communications (2021). DOI: 10.1038/s41467-021-25932-6 , www.nature.com/articles/s41467-021-25932-6


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Hexbyte Glen Cove New study measures neighborhood inequality and violence based on everyday mobility thumbnail

Hexbyte Glen Cove New study measures neighborhood inequality and violence based on everyday mobility

Hexbyte Glen Cove

Credit: Pixabay/CC0 Public Domain

A new study looking at the patterns of movement from 400,000 people offers fresh insights into how a neighborhood’s economic conditions mixed with the mobility patterns of its residents and visitors relates to the well-being of the neighborhood and can serve as a predictor of violence.

The analysis, published in the American Sociological Review, develops the concept of that have a “triple disadvantage.” These are neighborhoods that score high on common traits measuring “disadvantage”—such as concentrated poverty, unemployment, and how many residents receive public assistance—but also have deep connections with similarly disadvantaged neighborhoods through its resident’s own mobility and through mobility into the neighborhood from around the . The authors suggest these networks are formed through everyday movements, such as going to work, leisure activities, or visiting friends or family. Overall, the theory argues that a neighborhood’s well-being depends not only on its own socioeconomic conditions but on the conditions of the neighborhoods its residents visit and are visited by.

The findings underscore that neighborhoods, even ones distant from each other, are not islands in isolation but are inherently connected. In fact, the implications of triple disadvantaged neighborhoods are broad and potentially affect a wide range of issues, including community capacity, gentrification, transmission in a pandemic, and racial inequality.

“We’re trying to get researchers but also policymakers to think beyond just the characteristics of one neighborhood in isolation, which has driven a lot of research, including my own,” said Robert J. Sampson, the Henry Ford II Professor of the Social Sciences and author on the paper. “What we’re arguing is that triple disadvantage essentially exacerbates racial segregation [and other related factors]. … [It’s] the compounding of inequality by not just living in , but having disproportionate contact with other poor neighborhoods.”

The authors draw on a long tradition of research showing that violence is highly concentrated in certain neighborhoods and that a neighborhood’s poverty rate is strongly related to homicide. The researchers go beyond this traditional focus on residential areas and combine it with mobility data to study cross neighborhood ties and networks in entire cities.

Analyzing nearly 32,000 neighborhoods and 9,700 homicides in 37 of the largest U.S. cities, the authors show that triple disadvantage metrics can independently predict homicides after adjusting for known links of violence, such as density, race, age, and residential stability.

The authors look at neighborhoods in cities like New York City, Houston, Chicago, and many others, including smaller cities like Kansas City, Miami, Oakland, and Tulsa. Maps are available for all cities.

The researchers found that mobility-based disadvantage—meaning people flowing in and out of disadvantaged neighborhoods—accounts for roughly one-fifth of the relationship between residential disadvantage and homicide. Digging further, they saw that using measures of triple disadvantage rather than residential disadvantage increased the authors’ ability to predict neighborhood homicide counts by almost a third.

“The key there is that taking into account triple disadvantage, or taking into account these everyday mobility patterns, gives us added value in the prediction of homicide patterns,” Sampson said. “In other words, that it goes beyond just residential poverty—we show that there’s this additional explanatory value with respect to triple disadvantage.”

In the study, the authors also show what makes a neighborhood triply disadvantaged can swing the opposite way, too. When those from already advantaged neighborhoods visit and are visited by those from other advantaged neighborhoods, they become triply advantaged. This isolates and segregates affluent neighborhoods.

Working on the paper with Sampson was Brian L. Levy, an assistant professor at George Mason University, and Nolan E. Phillips, a data scientist at Accenture. Both were former postdoctoral fellows at Harvard.

The researchers say their work represents only the tip of the spear and builds on earlier work on segregation. They hope to expand their theory and have others use their methodologies, data, and new geographic data sources to run their own assessments.

“There’s a sense in which we hope that these ideas can be used by other researchers to create measures for studies around the world,” Sampson said. “Furthermore, we can imagine that researchers and even policymakers could create metrics for other kinds of indicators beyond what we started. … [The theory] has very expansive possibilities in our view.”

More information:
Brian L. Levy et al, Triple Disadvantage: Neighborhood Networks of Everyday Urban Mobility and Violence in U.S. Cities, American Sociological Review (2020). DOI: 10.1177/0003122420972323